bisect: add "git bisect help" subcommand to get a long usage string

Users are not often aware of the fact that "git bisect -h" can give
them a long usage description, as "git bisect" seems to accept only
dashless subcommands like "start", "good", ...

That's why this patch adds a "git bisect help" subcommand that just
calls "git bisect -h". This new subcommand is also fully documented
in the short usage string (that "git bisect" gives), in the long
usage string and in the man page (that "git help bisect" gives).

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-bisect.sh b/git-bisect.sh
index 48fb92d..a1343f6 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -1,7 +1,9 @@
 #!/bin/sh
 
-USAGE='[start|bad|good|skip|next|reset|visualize|replay|log|run]'
-LONG_USAGE='git bisect start [<bad> [<good>...]] [--] [<pathspec>...]
+USAGE='[help|start|bad|good|skip|next|reset|visualize|replay|log|run]'
+LONG_USAGE='git bisect help
+        print this long help message.
+git bisect start [<bad> [<good>...]] [--] [<pathspec>...]
         reset bisect state and start bisection.
 git bisect bad [<rev>]
         mark <rev> a known-bad revision.
@@ -20,7 +22,9 @@
 git bisect log
         show bisect log.
 git bisect run <cmd>...
-        use <cmd>... to automatically bisect.'
+        use <cmd>... to automatically bisect.
+
+Please use "git help bisect" to get the full man page.'
 
 OPTIONS_SPEC=
 . git-sh-setup
@@ -465,6 +469,8 @@
     cmd="$1"
     shift
     case "$cmd" in
+    help)
+        git bisect -h ;;
     start)
         bisect_start "$@" ;;
     bad|good|skip)