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>

2 files changed, 57 insertions, 0 deletions

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"])
+}