Support configurable git binary path for SSH remotes

Some SSH destinations may not expose 'git' on the non-interactive
shell's PATH, causing 'create project' and 'update head' operations
to fail. Add a new 'remote.NAME.gitPath' config option that lets
admins provide the absolute path to the 'git' binary on the remote
host.

Change-Id: I01e32cff5240cb55cf9e0fc0539344b340361ac4