rebase -i: introduce the 'break' command

The 'edit' command can be used to cherry-pick a commit and then
immediately drop out of the interactive rebase, with exit code 0, to let
the user amend the commit, or test it, or look around.

Sometimes this functionality would come in handy *without*
cherry-picking a commit, e.g. to interrupt the interactive rebase even
before cherry-picking a commit, or immediately after an 'exec' or a
'merge'.

This commit introduces that functionality, as the spanking new 'break'
command.

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

t/lib-rebase.sh

@@ -49,7 +49,7 @@ set_fake_editor () {
 		case $line in
 		squash|fixup|edit|reword|drop)
 			action="$line";;
-		exec*)
+		exec*|break)
 			echo "$line" | sed 's/_/ /g' >> "$1";;
 		"#")
 			echo '# comment' >> "$1";;