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>

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.

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"))
+}