Documentation/hooks: add pre-auto-gc hook

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/hooks.txt

diff --git a/Documentation/hooks.txt b/Documentation/hooks.txt
index 76b8d77..44fbe58 100644
--- a/Documentation/hooks.txt
+++ b/Documentation/hooks.txt
@@ -276,3 +276,10 @@
 Both standard output and standard error output are forwarded to
 `git-send-pack` on the other end, so you can simply `echo` messages
 for the user.
+
+pre-auto-gc
+-----------
+
+This hook is invoked by `git-gc --auto`. It takes no parameter, and
+exiting with non-zero status from this script causes the `git-gc --auto`
+to abort.