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/setting/git.go

@@ -87,7 +87,6 @@ func loadGitFrom(rootCfg ConfigProvider) {
 	GitConfig.Options = make(map[string]string)
 	GitConfig.SetOption("diff.algorithm", "histogram")
 	GitConfig.SetOption("core.logAllRefUpdates", "true")
-	GitConfig.SetOption("gc.reflogExpire", "90")
 
 	secGitReflog := rootCfg.Section("git.reflog")
 	if secGitReflog.HasKey("ENABLED") {
@@ -95,8 +94,8 @@ func loadGitFrom(rootCfg ConfigProvider) {
 		GitConfig.SetOption("core.logAllRefUpdates", secGitReflog.Key("ENABLED").In("true", []string{"true", "false"}))
 	}
 	if secGitReflog.HasKey("EXPIRATION") {
-		deprecatedSetting(rootCfg, "git.reflog", "EXPIRATION", "git.config", "core.reflogExpire", "1.21")
-		GitConfig.SetOption("gc.reflogExpire", secGitReflog.Key("EXPIRATION").String())
+		deprecatedSetting(rootCfg, "git.reflog", "EXPIRATION", "git.config", "gc.reflogExpire", "1.21")
+		GitConfig.SetOption("gc.reflogExpire", secGitReflog.Key("EXPIRATION").String()+".days")
 	}
 
 	for _, key := range secGitConfig.Keys() {