fix(git): leave reflog expiry to git's defaults (#39703) (#39711)

Backport #39703 by @silverwind

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

Co-authored-by: silverwind <me@silverwind.io>

modules/setting/git_test.go

@@ -38,8 +38,8 @@ diff.algorithm = other
 }
 
 func TestGitReflog(t *testing.T) {
-	defer test.MockVariableValue(&Git)
-	defer test.MockVariableValue(&GitConfig)
+	defer test.MockVariableValue(&Git)()
+	defer test.MockVariableValue(&GitConfig)()
 
 	// default reflog config without legacy options
 	cfg, err := NewConfigProviderFromData(``)
@@ -47,7 +47,7 @@ func TestGitReflog(t *testing.T) {
 	loadGitFrom(cfg)
 
 	assert.Equal(t, "true", GitConfig.GetOption("core.logAllRefUpdates"))
-	assert.Equal(t, "90", GitConfig.GetOption("gc.reflogExpire"))
+	assert.Empty(t, GitConfig.GetOption("gc.reflogExpire"))
 
 	// custom reflog config by legacy options
 	cfg, err = NewConfigProviderFromData(`
@@ -59,5 +59,5 @@ EXPIRATION = 123
 	loadGitFrom(cfg)
 
 	assert.Equal(t, "false", GitConfig.GetOption("core.logAllRefUpdates"))
-	assert.Equal(t, "123", GitConfig.GetOption("gc.reflogExpire"))
+	assert.Equal(t, "123.days", GitConfig.GetOption("gc.reflogExpire"))
 }