fix(git): leave reflog expiry to git's defaults (#39703)

Gitea's default `gc.reflogExpire = 90` is parsed by git as
`1990-<month>-<day>`, currently `1990-10-09`, so reachable reflog
entries never expired.

Since git 2.54, auto maintenance runs its reflog-expire task in the
foreground of every push, and its trigger ignores reachability, so busy
repos rerun `git reflog expire --all` on every push without pruning
anything, stalling large repos for over a minute.

- Stop setting `gc.reflogExpire` so git's own defaults apply, and remove
the `90` written by earlier versions
- Treat the legacy `[git.reflog] EXPIRATION` as days, as documented

The next push to each repo prunes the accumulated entries once.

Fixes: https://github.com/go-gitea/gitea/issues/39693

modules/git/config.go

@@ -119,6 +119,11 @@ func syncGitConfig(ctx context.Context) (err error) {
 		return err
 	}
 
+	// Gitea used to write "90", which git reads as a date in 1990
+	if err := configUnsetAll(ctx, "gc.reflogExpire", "90"); err != nil {
+		return err
+	}
+
 	// Apply user's git config options last so they take precedence over builtin defaults
 	for k, v := range setting.GitConfig.Options {
 		if err = configSet(ctx, strings.ToLower(k), v); err != nil {