commit 14a9bd2898b38449663644f862542d618f332952
author Jeff King <peff@peff.net> Thu May 31 18:42:53 2018 -0400
committer Junio C Hamano <gitster@pobox.com> Fri Jun 01 11:48:54 2018 +0900
tree 2ef15386854e782b2dc7e18904054fdd740af601
parent fc54c1af3ec09bab8b8ea09768c2da4069b7f53e

prepare_commit_graft: treat non-repository as a noop

The parse_commit_buffer() function consults lookup_commit_graft()
to see if we need to rewrite parents. The latter will look
at $GIT_DIR/info/grafts. If you're outside of a repository,
then this will trigger a BUG() as of b1ef400eec (setup_git_env:
avoid blind fall-back to ".git", 2016-10-20).

It's probably uncommon to actually parse a commit outside of
a repository, but you can see it in action with:

  cd /not/a/git/repo
  git index-pack --strict /some/file.pack

This works fine without --strict, but the fsck checks will
try to parse any commits, triggering the BUG(). We can fix
that by teaching the graft code to behave as if there are no
grafts when we aren't in a repository.

Arguably index-pack (and fsck) are wrong to consider grafts
at all. So another solution is to disable grafts entirely
for those commands. But given that the graft feature is
deprecated anyway, it's not worth even thinking through the
ramifications that might have.

There is one other corner case I considered here. What
should:

  cd /not/a/git/repo
  export GIT_GRAFT_FILE=/file/with/grafts
  git index-pack --strict /some/file.pack

do? We don't have a repository, but the user has pointed us
directly at a graft file, which we could respect. I believe
this case did work that way prior to b1ef400eec. However,
fixing it now would be pretty invasive. Back then we would
just call into setup_git_env() even without a repository.
But these days it actually takes a git_dir argument. So
there would be a fair bit of refactoring of the setup code
involved.

Given the obscurity of this case, plus the fact that grafts
are deprecated and probably shouldn't work under index-pack
anyway, it's not worth pursuing further. This patch at least
un-breaks the common case where you're _not_ using grafts,
but we BUG() anyway trying to even find that out.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>