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_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"])
+}