Don't use patches/<branch>/current

The name of the current patch, if any, is always the last line of
patches/<branch>/applied (and there is no current patch if and only if
the "applied" file is empty). So use that instead, and stop having to
worry about keeping the redundant "current" file up-to-date.

Signed-off-by: Karl Hasselström <kha@treskal.com>