Avoid importing invalid and duplicate patch names
This aims to be a comprehensive repair of issues with `stg import` allowing
invalid and duplicate patch names.
The new Patches.make_name() method creates a unique and valid patch name
based on its knowledge of existing patch names. Since StGit patch names are
used as Git refs, make_name() also attempts to enforce Git's rules for ref
naming. Generated patch names are checked with `git check-ref-format`
before being returned.
Repairs #64
Signed-off-by: Peter Grayson <pete@jpgrayson.net>