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, 29 insertions, 6 deletions

diff --git a/docs/content/doc/administration/config-cheat-sheet.en-us.md b/docs/content/doc/administration/config-cheat-sheet.en-us.md
index 349c480ae6..035c94cd27 100644
--- a/docs/content/doc/administration/config-cheat-sheet.en-us.md
+++ b/docs/content/doc/administration/config-cheat-sheet.en-us.md
@@ -1054,12 +1054,7 @@ Default templates for project boards:
 - `DISABLE_CORE_PROTECT_NTFS`: **false** Set to true to forcibly set `core.protectNTFS` to false.
 - `DISABLE_PARTIAL_CLONE`: **false** Disable the usage of using partial clones for git.
 
-## Git - Reflog settings (`git.reflog`)
-
-- `ENABLED`: **true** Set to true to enable Git to write changes to reflogs in each repo.
-- `EXPIRATION`: **90** Reflog entry lifetime, in days. Entries are removed opportunistically by Git.
-
-## Git - Timeout settings (`git.timeout`)
+### Git - Timeout settings (`git.timeout`)
 
 - `DEFAULT`: **360**: Git operations default timeout seconds.
 - `MIGRATE`: **600**: Migrate external repositories timeout seconds.
@@ -1068,6 +1063,18 @@ Default templates for project boards:
 - `PULL`: **300**: Git pull from internal repositories timeout seconds.
 - `GC`: **60**: Git repository GC timeout seconds.
 
+### Git - Reflog settings (`git.reflog`)
+
+- `ENABLED`: **true** Set to true to enable Git to write changes to reflogs in each repo.
+- `EXPIRATION`: **90** Reflog entry lifetime, in days. Entries are removed opportunistically by Git.
+
+### Git - Config options (`git.config`)
+
+The key/value pairs in this section will be used as git config.
+This section only does "set" config, a removed config key from this section won't be removed from git config automatically. The format is `some.configKey = value`.
+
+- `diff.algorithm`: **histogram**
+
 ## Metrics (`metrics`)
 
 - `ENABLED`: **false**: Enables /metrics endpoint for prometheus.
diff --git a/docs/content/doc/administration/customizing-gitea.en-us.md b/docs/content/doc/administration/customizing-gitea.en-us.md
index 54ce2a715f..4c8b1c90d7 100644
--- a/docs/content/doc/administration/customizing-gitea.en-us.md
+++ b/docs/content/doc/administration/customizing-gitea.en-us.md
@@ -282,6 +282,22 @@ Place custom files in corresponding sub-folder under `custom/options`.
 
 To add custom .gitignore, add a file with existing [.gitignore rules](https://git-scm.com/docs/gitignore) in it to `$GITEA_CUSTOM/options/gitignore`
 
+## Customizing the git configuration
+
+Starting with Gitea 1.20, you can customize the git configuration via the `git.config` section.
+
+### Enabling signed git pushes
+
+To enable signed git pushes, set these two options:
+
+```ini
+[git.config]
+receive.advertisePushOptions = true
+receive.certNonceSeed = <randomstring>
+```
+
+`certNonceSeed` should be set to a random string and be kept secret.
+
 ### Labels
 
 Starting with Gitea 1.19, you can add a file that follows the [YAML label format](https://github.com/go-gitea/gitea/blob/main/options/label/Advanced.yaml) to `$GITEA_CUSTOM/options/label`: