Look for templates in ~/.stgit/templates as well

This can be quite useful to avoid adding one's sig again and again to
ever covermail, and to use a patchmail template that insert
Signed-off-by lines, for those of us who prefer this to adding them to
commits.

Also make sure to use os.path.join() instead of hardcoded slashes.

Signed-off-by: Yann Dirson <ydirson@altern.org>