Add buttons to allow loading of incomplete diffs (#16829)

This PR adds two buttons to the stats and the end of the diffs list to load the (some of) the remaining incomplete diff sections.

Contains #16775
    
Signed-off-by: Andrew Thornton <art27@cantab.net>


## Screenshots

### Show more button at the end of the diff
![Screenshot from 2021-09-04 11-12-37](https://user-images.githubusercontent.com/1824502/132091009-b1f6113e-2c04-4be5-8a04-b8ecea56887b.png)

### Show more button at the end of the diff stats box
![Screenshot from 2021-09-04 11-14-54](https://user-images.githubusercontent.com/1824502/132091063-86da5a6d-6628-4b82-bea9-3655cd9f40f6.png)

3 files changed, 27 insertions, 11 deletions

diff --git a/services/gitdiff/gitdiff.go b/services/gitdiff/gitdiff.go
index ef66675c5b..f843bc4dcf 100644
--- a/services/gitdiff/gitdiff.go
+++ b/services/gitdiff/gitdiff.go
@@ -653,6 +653,7 @@ func getCommitFileLineCount(commit *git.Commit, filePath string) int {
 
 // Diff represents a difference between two git trees.
 type Diff struct {
+	Start, End                             string
 	NumFiles, TotalAddition, TotalDeletion int
 	Files                                  []*DiffFile
 	IsIncomplete                           bool
@@ -719,6 +720,9 @@ parsingLoop:
 
 		// TODO: Handle skipping first n files
 		if len(diff.Files) >= maxFiles {
+
+			lastFile := createDiffFile(diff, line)
+			diff.End = lastFile.Name
 			diff.IsIncomplete = true
 			_, err := io.Copy(io.Discard, reader)
 			if err != nil {
@@ -1217,7 +1221,7 @@ func readFileName(rd *strings.Reader) (string, bool) {
 // GetDiffRangeWithWhitespaceBehavior builds a Diff between two commits of a repository.
 // Passing the empty string as beforeCommitID returns a diff from the parent commit.
 // The whitespaceBehavior is either an empty string or a git flag
-func GetDiffRangeWithWhitespaceBehavior(gitRepo *git.Repository, beforeCommitID, afterCommitID string, maxLines, maxLineCharacters, maxFiles int, whitespaceBehavior string, directComparison bool) (*Diff, error) {
+func GetDiffRangeWithWhitespaceBehavior(gitRepo *git.Repository, beforeCommitID, afterCommitID, skipTo string, maxLines, maxLineCharacters, maxFiles int, whitespaceBehavior string, directComparison bool) (*Diff, error) {
 	repoPath := gitRepo.Path
 
 	commit, err := gitRepo.GetCommit(afterCommitID)
@@ -1228,31 +1232,42 @@ func GetDiffRangeWithWhitespaceBehavior(gitRepo *git.Repository, beforeCommitID,
 	ctx, cancel := context.WithTimeout(git.DefaultContext, time.Duration(setting.Git.Timeout.Default)*time.Second)
 	defer cancel()
 
-	var cmd *exec.Cmd
+	argsLength := 6
+	if len(whitespaceBehavior) > 0 {
+		argsLength++
+	}
+	if len(skipTo) > 0 {
+		argsLength++
+	}
+
+	diffArgs := make([]string, 0, argsLength)
 	if (len(beforeCommitID) == 0 || beforeCommitID == git.EmptySHA) && commit.ParentCount() == 0 {
-		diffArgs := []string{"diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M"}
+		diffArgs = append(diffArgs, "diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M")
 		if len(whitespaceBehavior) != 0 {
 			diffArgs = append(diffArgs, whitespaceBehavior)
 		}
 		// append empty tree ref
 		diffArgs = append(diffArgs, "4b825dc642cb6eb9a060e54bf8d69288fbee4904")
 		diffArgs = append(diffArgs, afterCommitID)
-		cmd = exec.CommandContext(ctx, git.GitExecutable, diffArgs...)
 	} else {
 		actualBeforeCommitID := beforeCommitID
 		if len(actualBeforeCommitID) == 0 {
 			parentCommit, _ := commit.Parent(0)
 			actualBeforeCommitID = parentCommit.ID.String()
 		}
-		diffArgs := []string{"diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M"}
+		diffArgs = append(diffArgs, "diff", "--src-prefix=\\a/", "--dst-prefix=\\b/", "-M")
 		if len(whitespaceBehavior) != 0 {
 			diffArgs = append(diffArgs, whitespaceBehavior)
 		}
 		diffArgs = append(diffArgs, actualBeforeCommitID)
 		diffArgs = append(diffArgs, afterCommitID)
-		cmd = exec.CommandContext(ctx, git.GitExecutable, diffArgs...)
 		beforeCommitID = actualBeforeCommitID
 	}
+	if skipTo != "" {
+		diffArgs = append(diffArgs, "--skip-to="+skipTo)
+	}
+	cmd := exec.CommandContext(ctx, git.GitExecutable, diffArgs...)
+
 	cmd.Dir = repoPath
 	cmd.Stderr = os.Stderr
 
@@ -1272,6 +1287,7 @@ func GetDiffRangeWithWhitespaceBehavior(gitRepo *git.Repository, beforeCommitID,
 	if err != nil {
 		return nil, fmt.Errorf("ParsePatch: %v", err)
 	}
+	diff.Start = skipTo
 
 	var checker *git.CheckAttributeReader
 
@@ -1299,7 +1315,7 @@ func GetDiffRangeWithWhitespaceBehavior(gitRepo *git.Repository, beforeCommitID,
 				log.Error("Unable to open checker for %s. Error: %v", afterCommitID, err)
 			} else {
 				go func() {
-					err = checker.Run()
+					err := checker.Run()
 					if err != nil && err != ctx.Err() {
 						log.Error("Unable to open checker for %s. Error: %v", afterCommitID, err)
 					}
@@ -1382,8 +1398,8 @@ func GetDiffRangeWithWhitespaceBehavior(gitRepo *git.Repository, beforeCommitID,
 
 // GetDiffCommitWithWhitespaceBehavior builds a Diff representing the given commitID.
 // The whitespaceBehavior is either an empty string or a git flag
-func GetDiffCommitWithWhitespaceBehavior(gitRepo *git.Repository, commitID string, maxLines, maxLineCharacters, maxFiles int, whitespaceBehavior string, directComparison bool) (*Diff, error) {
-	return GetDiffRangeWithWhitespaceBehavior(gitRepo, "", commitID, maxLines, maxLineCharacters, maxFiles, whitespaceBehavior, directComparison)
+func GetDiffCommitWithWhitespaceBehavior(gitRepo *git.Repository, commitID, skipTo string, maxLines, maxLineCharacters, maxFiles int, whitespaceBehavior string, directComparison bool) (*Diff, error) {
+	return GetDiffRangeWithWhitespaceBehavior(gitRepo, "", commitID, skipTo, maxLines, maxLineCharacters, maxFiles, whitespaceBehavior, directComparison)
 }
 
 // CommentAsDiff returns c.Patch as *Diff
diff --git a/services/gitdiff/gitdiff_test.go b/services/gitdiff/gitdiff_test.go
index 0c216ccb54..d69d0c01d8 100644
--- a/services/gitdiff/gitdiff_test.go
+++ b/services/gitdiff/gitdiff_test.go
@@ -522,7 +522,7 @@ func TestGetDiffRangeWithWhitespaceBehavior(t *testing.T) {
 	}
 	defer gitRepo.Close()
 	for _, behavior := range []string{"-w", "--ignore-space-at-eol", "-b", ""} {
-		diffs, err := GetDiffRangeWithWhitespaceBehavior(gitRepo, "559c156f8e0178b71cb44355428f24001b08fc68", "bd7063cc7c04689c4d082183d32a604ed27a24f9",
+		diffs, err := GetDiffRangeWithWhitespaceBehavior(gitRepo, "559c156f8e0178b71cb44355428f24001b08fc68", "bd7063cc7c04689c4d082183d32a604ed27a24f9", "",
 			setting.Git.MaxGitDiffLines, setting.Git.MaxGitDiffLines, setting.Git.MaxGitDiffFiles, behavior, false)
 		assert.NoError(t, err, fmt.Sprintf("Error when diff with %s", behavior))
 		for _, f := range diffs.Files {
diff --git a/services/pull/pull.go b/services/pull/pull.go
index f7d154cfd0..5f6198a393 100644
--- a/services/pull/pull.go
+++ b/services/pull/pull.go
@@ -80,7 +80,7 @@ func NewPullRequest(repo *models.Repository, pull *models.Issue, labelIDs []int6
 	defer baseGitRepo.Close()
 
 	compareInfo, err := baseGitRepo.GetCompareInfo(pr.BaseRepo.RepoPath(),
-		git.BranchPrefix+pr.BaseBranch, pr.GetGitRefName(), true)
+		git.BranchPrefix+pr.BaseBranch, pr.GetGitRefName(), true, false)
 	if err != nil {
 		return err
 	}