"git rev-parse --git-dir" fails from inside .git/<any-subdir>

[Jean-Luc Herren <jlh@gmx.ch>]

Hello list!

I noticed that "git rev-parse --git-dir" doesn't work properly
when called from inside a sub-directory of .git.  It seems
setup_git_directory_gently() is the problem, because it simply
calls chdir("..") until "." is a valid git directory and it then
sets GIT_DIR = ".".  But this relative path only works as long as
you stay in the directory that setup_git_directory_gently() has
put you in.  It won't work when git-sh-setup uses GIT_DIR=$(git
rev-parse --git-dir).

This causes "git repack -a -f" to fail with "Nothing new to pack."
from inside .git/objects/pack, which would be handy to test
different parameters and compare the resulting pack files.

I don't know whether setup_git_directory_gently() or rather
builtin-rev-parse.c needs to be changed, I'll let someone else do
this as I don't know the code base well enough.

jlh