Handle branch names with slashes

Teach stgit to handle branch names with slashes in them; that is,
branches living in a subdirectory of .git/refs/heads.

I had to change the patch@branch/top command-line syntax to
patch@branch//top, in order to get sane parsing. The /top variant is
still available for repositories that have no slashy branches; it is
disabled as soon as there exists at least one subdirectory of
refs/heads. Preferably, this compatibility hack can be killed some
time in the future.

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