http-fetch: set up git directory before parsing pack hashes

http-fetch: set up git directory before parsing pack hashes

In dd4b732df7 ("upload-pack: send part of packfile response as uri",
2020-06-10), the git http-fetch code learned how to take  ac --packfile
option.  This option takes an argument, which is the name of a packfile
hash, and parses it using parse_oid_hex.  It does so before calling
setup_git_directory.

However, in a SHA-256 repository this fails to work, since we have not
set the hash algorithm in use and parse_oid_hex fails as a consequence.
To ensure that we can parse packfile hashes of the right length, let's
set up the git directory before we start parsing arguments.

Since we still want to allow the invocation of -h to print the help when
we're not in a repository, gracefully handle us being outside of one and
produce an error after argument parsing has finished.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>