enhance!: raise minimum git version to 2.34 (#39565)

Raise the minimum git version to 2.34, the version in Ubuntu 22.04,
Debian 12 and RHEL 8 ship newer, and remove the fallbacks it makes
obsolete.

- Always enable AGit
- Use `diff --skip-to` and `apply -3` unconditionally
- Set the default branch of new repos and wikis via `git init
--initial-branch`
- Detect rebase conflicts via `REBASE_HEAD`

modules/git/repo.go

@@ -118,7 +118,7 @@ func IsRepoURLAccessible(ctx context.Context, url string) bool {
 }
 
 // InitRepositoryLocal initializes a new Git repository.
-func InitRepositoryLocal(ctx context.Context, localRepoPath string, bare bool, objectFormatName string) error {
+func InitRepositoryLocal(ctx context.Context, localRepoPath string, bare bool, objectFormatName, initialBranch string) error {
 	err := os.MkdirAll(localRepoPath, os.ModePerm)
 	if err != nil {
 		return err
@@ -132,6 +132,9 @@ func InitRepositoryLocal(ctx context.Context, localRepoPath string, bare bool, o
 	if DefaultFeatures().SupportHashSha256 {
 		cmd.AddOptionValues("--object-format", objectFormatName)
 	}
+	if initialBranch != "" {
+		cmd.AddOptionValues("--initial-branch", initialBranch)
+	}
 
 	if bare {
 		cmd.AddArguments("--bare")