fix extra newlines when copying from diff in Firefox (#7288)

* fix extra newlines when copying from diff

See https://bugzilla.mozilla.org/show_bug.cgi?id=1273836

Basically, the <pre><code> seems to add a forced newline that is not
possible to get rid of via CSS, so I replaced it with just a <code>.

Secondly, .lines-type-marker also forced a newline in the copied text,
but that was possible to get rid of via user-select.

Safari still has a extraneous newline in the copied text of unknown
origin, but this should not block stop this PR.

* simplify .line-type-marker

* fix selector

* remove erronous ^^^

* Fix empty split diff

* Fix arc-theme-green

* fix add comment

* ensure line-num is copied too

* Update templates/repo/diff/box.tmpl

Co-Authored-By: zeripath <art27@cantab.net>

* attempt to fix safari via removing <code>

* remove useless whitespace at the end of 'class'

* remove inter-tag whitespace for code <td>s

* more inter-tag removal

* final inter-tag removal

* attempt to fix empty line copy

* move and comment getLineContent

* fix golint

* make background grey for missing added code

1 files changed, 16 insertions, 8 deletions

diff --git a/models/git_diff.go b/models/git_diff.go
index bc79a73a4e..2f48f1b6fa 100644
--- a/models/git_diff.go
+++ b/models/git_diff.go
@@ -88,6 +88,14 @@ func (d *DiffLine) GetLineTypeMarker() string {
 	return ""
 }
 
+// escape a line's content or return <br> needed for copy/paste purposes
+func getLineContent(content string) string {
+	if len(content) > 0 {
+		return html.EscapeString(content)
+	}
+	return "<br>"
+}
+
 // DiffSection represents a section of a DiffFile.
 type DiffSection struct {
 	Name  string
@@ -107,14 +115,14 @@ func diffToHTML(diffs []diffmatchpatch.Diff, lineType DiffLineType) template.HTM
 		switch {
 		case diffs[i].Type == diffmatchpatch.DiffInsert && lineType == DiffLineAdd:
 			buf.Write(addedCodePrefix)
-			buf.WriteString(html.EscapeString(diffs[i].Text))
+			buf.WriteString(getLineContent(diffs[i].Text))
 			buf.Write(codeTagSuffix)
 		case diffs[i].Type == diffmatchpatch.DiffDelete && lineType == DiffLineDel:
 			buf.Write(removedCodePrefix)
-			buf.WriteString(html.EscapeString(diffs[i].Text))
+			buf.WriteString(getLineContent(diffs[i].Text))
 			buf.Write(codeTagSuffix)
 		case diffs[i].Type == diffmatchpatch.DiffEqual:
-			buf.WriteString(html.EscapeString(diffs[i].Text))
+			buf.WriteString(getLineContent(diffs[i].Text))
 		}
 	}
 
@@ -173,7 +181,7 @@ func init() {
 // GetComputedInlineDiffFor computes inline diff for the given line.
 func (diffSection *DiffSection) GetComputedInlineDiffFor(diffLine *DiffLine) template.HTML {
 	if setting.Git.DisableDiffHighlight {
-		return template.HTML(html.EscapeString(diffLine.Content[1:]))
+		return template.HTML(getLineContent(diffLine.Content[1:]))
 	}
 	var (
 		compareDiffLine *DiffLine
@@ -186,22 +194,22 @@ func (diffSection *DiffSection) GetComputedInlineDiffFor(diffLine *DiffLine) tem
 	case DiffLineAdd:
 		compareDiffLine = diffSection.GetLine(DiffLineDel, diffLine.RightIdx)
 		if compareDiffLine == nil {
-			return template.HTML(html.EscapeString(diffLine.Content[1:]))
+			return template.HTML(getLineContent(diffLine.Content[1:]))
 		}
 		diff1 = compareDiffLine.Content
 		diff2 = diffLine.Content
 	case DiffLineDel:
 		compareDiffLine = diffSection.GetLine(DiffLineAdd, diffLine.LeftIdx)
 		if compareDiffLine == nil {
-			return template.HTML(html.EscapeString(diffLine.Content[1:]))
+			return template.HTML(getLineContent(diffLine.Content[1:]))
 		}
 		diff1 = diffLine.Content
 		diff2 = compareDiffLine.Content
 	default:
 		if strings.IndexByte(" +-", diffLine.Content[0]) > -1 {
-			return template.HTML(html.EscapeString(diffLine.Content[1:]))
+			return template.HTML(getLineContent(diffLine.Content[1:]))
 		}
-		return template.HTML(html.EscapeString(diffLine.Content))
+		return template.HTML(getLineContent(diffLine.Content))
 	}
 
 	diffRecord := diffMatchPatch.DiffMain(diff1[1:], diff2[1:], true)