rebase-i: print an abbreviated hash when stop for editing

The message that is shown when rebase-i stops for editing prints
the full hash of the commit where it stopped which makes the message
overflow to the next line on smaller terminal windows.  Print an
abbreviated hash instead.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ralfth

git-rebase--interactive.sh

@@ -552,7 +552,8 @@ do_next () {
 
 		mark_action_done
 		do_pick $sha1 "$rest"
-		warn "Stopped at $sha1... $rest"
+		sha1_abbrev=$(git rev-parse --short $sha1)
+		warn "Stopped at $sha1_abbrev... $rest"
 		exit_with_patch $sha1 0
 		;;
 	squash|s|fixup|f)