socorro-syms: Limit the search for repository to within the build directory

When we search for the git repository associated with a file and the there's no
repository associated, we incorrectly find the yocto repository itself.

To prevent the search from finding an incorrect repository, we only accept
repositories found within the build directory.

Signed-off-by: Daniel Markus <daniel.markus@leica-geosystems.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>