fix(git): default to the gc maintenance strategy (#40022) (#40029)

Backport #40022 by @silverwind

Git 2.54 made "geometric" the default auto-maintenance strategy. Pushes
wait for its foreground reflog expiry
(https://github.com/go-gitea/gitea/issues/39693), and its detached
repacks race with filesystem operations, as in
https://github.com/go-gitea/gitea/actions/runs/40236681252/job/121299686933.
Pin the pre-2.54 "gc" strategy, older git ignores it.

https://github.com/go-gitea/gitea/pull/39745 is the more complete fix
which disables the auto-maintenance.

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

modules/setting/git_test.go

@@ -27,6 +27,7 @@ a.b = 1
 	loadGitFrom(cfg)
 	assert.Equal(t, "1", GitConfig.Options["a.b"])
 	assert.Equal(t, "histogram", GitConfig.Options["diff.algorithm"])
+	assert.Equal(t, "gc", GitConfig.Options["maintenance.strategy"])
 
 	cfg, err = NewConfigProviderFromData(`
 [git.config]