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/git/config.go

@@ -125,6 +125,11 @@ 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 {

modules/git/config_test.go

@@ -60,8 +60,10 @@ func TestSyncGitConfig(t *testing.T) {
 	defer test.MockVariableValue(&setting.GitConfig)()
 
 	assert.Empty(t, setting.GitConfig.Options)
+	assert.NoError(t, configSet(t.Context(), "gc.reflogExpire", "90"))
 	assert.NoError(t, syncGitConfig(t.Context()))
 	assert.True(t, gitConfigContains("commitGraph = true")) // builtin default config
+	assert.False(t, gitConfigContains("reflogExpire"))
 
 	setting.GitConfig.Options["sync-test.cfg-key-a"] = "CfgValA"
 	setting.GitConfig.Options["core.commitgraph"] = "false"