has_sha1_file: don't bother if we are not in a repository

Most callers to this function already require that they are in a
git repository, but there is an exception: "git apply" uses
has_sha1_file to avoid work if the result of applying a binary
patch is already present in the repository. When run outside any
repository, this produces an error:

 fatal: BUG: setup_git_env called without repository

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -3320,6 +3320,8 @@ int has_sha1_file_with_flags(const unsigned char *sha1, int flags)
 {
 	struct pack_entry e;
 
+	if (!startup_info->have_repository)
+		return 0;
 	if (find_pack_entry(sha1, &e))
 		return 1;
 	if (has_loose_object(sha1))