enhance(repo): add default object format setting (#38877)

Adds `[repository] DEFAULT_OBJECT_FORMAT` to default new repositories to
`sha1` or `sha256`.

Applies the setting to repository creation defaults in the UI and API,
reducing repeated manual selection.

Docs: https://gitea.com/gitea/docs/pulls/504

Fixes https://github.com/go-gitea/gitea/issues/38854

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/repository.go

@@ -57,6 +57,7 @@ var (
 		DisableMigrations                       bool
 		DisableStars                            bool `ini:"DISABLE_STARS"`
 		DefaultBranch                           string
+		DefaultObjectFormat                     string
 		AllowAdoptionOfUnadoptedRepositories    bool
 		AllowDeleteOfUnadoptedRepositories      bool
 		DisableDownloadSourceArchives           bool
@@ -186,6 +187,7 @@ var (
 		DisableMigrations:                       false,
 		DisableStars:                            false,
 		DefaultBranch:                           "main",
+		DefaultObjectFormat:                     "sha1",
 		AllowForkWithoutMaximumLimit:            true,
 		StreamArchives:                          true,