completion: audit and guard $GIT_* against unset use

completion: audit and guard $GIT_* against unset use

$GIT_COMPLETION_SHOW_ALL and $GIT_TESTING_ALL_COMMAND_LIST were used
without guarding against them being unset, causing errors in nounset
(set -u) mode.

No other nounset-unsafe $GIT_* usages were found.

While at it, remove a superfluous (duplicate) unset guard from $GIT_DIR
in __git_find_repo_path.

Signed-off-by: Ville Skyttä <ville.skytta@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>