Add a subclassed edit box that verifies the existence of a
user-specified git repo.
Also adds a verify_git_repo() function that can be used as a basic
sanity check for local git setup.
(From meta-yocto rev: 64f1176d62ed02d7c61d32fdae11dc8b7d365603)
Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>