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/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)