set_git_dir: die when setenv() fails

set_git_dir: die when setenv() fails

The set_git_dir() function returns an error if setenv()
fails, but there are zero callers who pay attention to this
return value. If this ever were to happen, it could cause
confusing results, as sub-processes would see a potentially
stale GIT_DIR (e.g., if it is relative and we chdir()-ed to
the root of the working tree).

We _could_ try to fix each caller, but there's really
nothing useful to do after this failure except die. Let's
just lump setenv() failure into the same category as malloc
failure: things that should never happen and cause us to
abort catastrophically.

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