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

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>