cat_file: take -i/-w parameters

cat_file: take -i/-w parameters

This changes the way how cat_file selects the source of the file. It
accepts an optional parameter which is either -i or -w and will react on this
instead of the branch name. tg-patch is updated accordingly and can now
accepts the current branch name as argument with -i or -w given.

cat_file was also broken for the worktree case when we are not in the top level.

Also, tg-patch allowed to be on the top-base branch, but -i and -w doesn't
make sense there too.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>