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>

rebase-interactive.c

@@ -14,6 +14,7 @@ void append_todo_help(unsigned edit_todo, unsigned keep_empty,
 "s, squash <commit> = use commit, but meld into previous commit\n"
 "f, fixup <commit> = like \"squash\", but discard this commit's log message\n"
 "x, exec <command> = run command (the rest of the line) using shell\n"
+"b, break = stop here (continue rebase later with 'git rebase --continue')\n"
 "d, drop <commit> = remove commit\n"
 "l, label <label> = label current HEAD with a name\n"
 "t, reset <label> = reset HEAD to a label\n"