Support changing git config through `app.ini`, use `diff.algorithm=histogram` by default (#24860)

Close #13454 , Close #23255, Close #14697 (and maybe more related
issues)

Many users have the requirement to customize the git config. This PR
introduces an easy way: put the options in Gitea's app.ini
`[git.config]`, then the config options will be applied to git config.

And it can support more flexible default config values, eg: now
`diff.algorithm=histogram` by default. According to:
https://stackoverflow.com/a/32367597/4754037 , `histogram diff` is
efficient and doesn't like to cause server-side problems.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: Giteabot <teabot@gitea.io>

4 files changed, 83 insertions, 6 deletions

diff --git a/modules/git/git.go b/modules/git/git.go
index a31afc077a..2e0a16fb5c 100644
--- a/modules/git/git.go
+++ b/modules/git/git.go
@@ -224,6 +224,14 @@ func syncGitConfig() (err error) {
 		return fmt.Errorf("unable to prepare git home directory %s, err: %w", HomeDir(), err)
 	}
 
+	// first, write user's git config options to git config file
+	// user config options could be overwritten by builtin values later, because if a value is builtin, it must have some special purposes
+	for k, v := range setting.GitConfig.Options {
+		if err = configSet(strings.ToLower(k), v); err != nil {
+			return err
+		}
+	}
+
 	// Git requires setting user.name and user.email in order to commit changes - old comment: "if they're not set just add some defaults"
 	// TODO: need to confirm whether users really need to change these values manually. It seems that these values are dummy only and not really used.
 	// If these values are not really used, then they can be set (overwritten) directly without considering about existence.
diff --git a/modules/git/git_test.go b/modules/git/git_test.go
index 25eb308531..37ab669ea4 100644
--- a/modules/git/git_test.go
+++ b/modules/git/git_test.go
@@ -42,14 +42,14 @@ func TestMain(m *testing.M) {
 	}
 }
 
-func TestGitConfig(t *testing.T) {
-	gitConfigContains := func(sub string) bool {
-		if b, err := os.ReadFile(HomeDir() + "/.gitconfig"); err == nil {
-			return strings.Contains(string(b), sub)
-		}
-		return false
+func gitConfigContains(sub string) bool {
+	if b, err := os.ReadFile(HomeDir() + "/.gitconfig"); err == nil {
+		return strings.Contains(string(b), sub)
 	}
+	return false
+}
 
+func TestGitConfig(t *testing.T) {
 	assert.False(t, gitConfigContains("key-a"))
 
 	assert.NoError(t, configSetNonExist("test.key-a", "val-a"))
@@ -81,3 +81,15 @@ func TestGitConfig(t *testing.T) {
 	assert.NoError(t, configUnsetAll("test.key-x", "*"))
 	assert.False(t, gitConfigContains("key-x = *"))
 }
+
+func TestSyncConfig(t *testing.T) {
+	oldGitConfig := setting.GitConfig
+	defer func() {
+		setting.GitConfig = oldGitConfig
+	}()
+
+	setting.GitConfig.Options["sync-test.cfg-key-a"] = "CfgValA"
+	assert.NoError(t, syncGitConfig())
+	assert.True(t, gitConfigContains("[sync-test]"))
+	assert.True(t, gitConfigContains("cfg-key-a = CfgValA"))
+}
diff --git a/modules/setting/git.go b/modules/setting/git.go
index b8e7bb9cf8..29ec37f866 100644
--- a/modules/setting/git.go
+++ b/modules/setting/git.go
@@ -5,6 +5,7 @@ package setting
 
 import (
 	"path/filepath"
+	"strings"
 	"time"
 
 	"code.gitea.io/gitea/modules/log"
@@ -78,12 +79,28 @@ var Git = struct {
 	},
 }
 
+var GitConfig = struct {
+	Options map[string]string
+}{
+	Options: make(map[string]string),
+}
+
 func loadGitFrom(rootCfg ConfigProvider) {
 	sec := rootCfg.Section("git")
 	if err := sec.MapTo(&Git); err != nil {
 		log.Fatal("Failed to map Git settings: %v", err)
 	}
 
+	secGitConfig := rootCfg.Section("git.config")
+	GitConfig.Options = make(map[string]string)
+	for _, key := range secGitConfig.Keys() {
+		// git config key is case-insensitive, so always use lower-case
+		GitConfig.Options[strings.ToLower(key.Name())] = key.String()
+	}
+	if _, ok := GitConfig.Options["diff.algorithm"]; !ok {
+		GitConfig.Options["diff.algorithm"] = "histogram"
+	}
+
 	Git.HomePath = sec.Key("HOME_PATH").MustString("home")
 	if !filepath.IsAbs(Git.HomePath) {
 		Git.HomePath = filepath.Join(AppDataPath, Git.HomePath)
diff --git a/modules/setting/git_test.go b/modules/setting/git_test.go
new file mode 100644
index 0000000000..1da8c87cc8
--- /dev/null
+++ b/modules/setting/git_test.go
@@ -0,0 +1,40 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGitConfig(t *testing.T) {
+	oldGit := Git
+	oldGitConfig := GitConfig
+	defer func() {
+		Git = oldGit
+		GitConfig = oldGitConfig
+	}()
+
+	cfg, err := NewConfigProviderFromData(`
+[git.config]
+a.b = 1
+`)
+	assert.NoError(t, err)
+	loadGitFrom(cfg)
+
+	assert.Len(t, GitConfig.Options, 2)
+	assert.EqualValues(t, "1", GitConfig.Options["a.b"])
+	assert.EqualValues(t, "histogram", GitConfig.Options["diff.algorithm"])
+
+	cfg, err = NewConfigProviderFromData(`
+[git.config]
+diff.algorithm = other
+`)
+	assert.NoError(t, err)
+	loadGitFrom(cfg)
+
+	assert.Len(t, GitConfig.Options, 1)
+	assert.EqualValues(t, "other", GitConfig.Options["diff.algorithm"])
+}