Check git version for --branch support. Fixes #2504
Buildslave fires SIGTERM before SIGKILL

Before forcefully killing the process, we fire a SIGTERM in case
the developer wants to capture some debugging information.
Updates test_simple to check if process was killed by SIGTERM
Adds test_kill to buildslave tests

Adds a unit test which ensures the process will receive SIGKILL if
SIGTERM fails to end the process.