setup_git_directory_gently_1(): avoid die()ing

setup_git_directory_gently_1(): avoid die()ing

This function now has a new caller in addition to setup_git_directory():
the newly introduced discover_git_directory(). That function wants to
discover the current .git/ directory, and in case of a corrupted one
simply pretend that there is none to be found.

Example: if a stale .git file exists in the parent directory, and the
user calls `git -p init`, we want Git to simply *not* read any
repository config for the pager (instead of aborting with a message that
the .git file is corrupt).

Let's actually pretend that there was no GIT_DIR to be found in that case
when being called from discover_git_directory(), but keep the previous
behavior (i.e. to die()) for the setup_git_directory() case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>