git-difftool--helper: honor --trust-exit-code with --dir-diff

The `--trust-exit-code` option for git-diff-tool(1) was introduced via
2b52123fcf (difftool: add support for --trust-exit-code, 2014-10-26).
When set, it makes us return the exit code of the invoked diff tool when
diffing multiple files. This patch didn't change the code path where
`--dir-diff` was passed because we already returned the exit code of the
diff tool unconditionally in that case.

This was changed a month later via c41d3fedd8 (difftool--helper: add
explicit exit statement, 2014-11-20), where an explicit `exit 0` was
added to the end of git-difftool--helper.sh. While the stated intent of
that commit was merely a cleanup, it had the consequence that we now
to ignore the exit code of the diff tool when `--dir-diff` was set. This
change in behaviour is thus very likely an unintended side effect of
this patch.

Now there are two ways to fix this:

  - We can either restore the original behaviour, which unconditionally
    returned the exit code of the diffing tool when `--dir-diff` is
    passed.

  - Or we can make the `--dir-diff` case respect the `--trust-exit-code`
    flag.

The fact that we have been ignoring exit codes for 7 years by now makes
me rather lean towards the latter option. Furthermore, respecting the
flag in one case but not the other would needlessly make the user
interface more complex.

Fix the bug so that we also honor `--trust-exit-code` for dir diffs and
adjust the documentation accordingly.

Reported-by: Jean-Rémy Falleri <jr.falleri@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -91,6 +91,19 @@ then
 	# ignore the error from the above --- run_merge_tool
 	# will diagnose unusable tool by itself
 	run_merge_tool "$merge_tool" false
+
+	status=$?
+	if test $status -ge 126
+	then
+		# Command not found (127), not executable (126) or
+		# exited via a signal (>= 128).
+		exit $status
+	fi
+
+	if test "$GIT_DIFFTOOL_TRUST_EXIT_CODE" = true
+	then
+		exit $status
+	fi
 else
 	# Launch the merge tool on each path provided by 'git diff'
 	while test $# -gt 6