rebase -i: support single-letter abbreviations for the actions

When you do many rebases, you can get annoyed by having to type out
the actions "edit" or "squash" in total.

This commit helps that, by allowing you to enter "e" instead of "edit",
"p" instead of "pick", or "s" instead of "squash".

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -232,14 +232,14 @@ do_next () {
 	'#'*|'')
 		mark_action_done
 		;;
-	pick)
+	pick|p)
 		comment_for_reflog pick
 
 		mark_action_done
 		pick_one $sha1 ||
 			die_with_patch $sha1 "Could not apply $sha1... $rest"
 		;;
-	edit)
+	edit|e)
 		comment_for_reflog edit
 
 		mark_action_done
@@ -254,7 +254,7 @@ do_next () {
 		warn
 		exit 0
 		;;
-	squash)
+	squash|s)
 		comment_for_reflog squash
 
 		has_action "$DONE" ||
@@ -263,7 +263,7 @@ do_next () {
 		mark_action_done
 		make_squash_message $sha1 > "$MSG"
 		case "$(peek_next_command)" in
-		squash)
+		squash|s)
 			EDIT_COMMIT=
 			USE_OUTPUT=output
 			cp "$MSG" "$SQUASH_MSG"