fix: preserve object format dropdown options on /repo/create error (#4360)

To reproduce:
- make the repo creation form return with an error, like a duplicate name
- click on the Object format dropdown
- the options are missing as the listbox is empty

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4360
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Solomon Victorino <git@solomonvictorino.com>
Co-committed-by: Solomon Victorino <git@solomonvictorino.com>

routers/web/repo/repo.go

@@ -231,6 +231,8 @@ func CreatePost(ctx *context.Context) {
 
 	ctx.Data["CanCreateRepo"] = ctx.Doer.CanCreateRepo()
 	ctx.Data["MaxCreationLimit"] = ctx.Doer.MaxCreationLimit()
+	ctx.Data["SupportedObjectFormats"] = git.SupportedObjectFormats
+	ctx.Data["DefaultObjectFormat"] = git.Sha1ObjectFormat
 
 	ctxUser := checkContextUser(ctx, form.UID)
 	if ctx.Written() {