setenv(GIT_DIR) clean-up

setenv(GIT_DIR) clean-up

This patch converts the setenv() calls in path.c and setup.c.  After
the call, git grep with a pager works again in bare repos.

It leaves the setenv(GIT_DIR_ENVIRONMENT, ...) calls in git.c alone, as
they respond to command line switches that emulate the effect of setting
the environment variable directly.

The remaining site in environment.c is in set_git_dir() and is left
alone, too, of course.  Finally, builtin-init-db.c is left changed
because the repo is still being carefully constructed when the
environment variable is set.

This fixes git shortlog when run inside a git directory, which had been
broken by abe549e1.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>