Rebuilding Gitaly is done for a Gitaly version that mismatches the
GITALY_SERVER_VERSION. Due to a continuous deployment, these rebuilds
are quite frequent and thus triggered a lot.

This change allows a newer Gitaly to be used, locally, for these tests.
Which in practice will decrease the number of rebuilds.

This achieved by running `git merge-base --is-ancestor` in the tmp
component directory, and depends on the HEAD being the same as the built
assets.

The other option was to parse `gitaly -version`, but this wouldn't have
worked for `gitlab-shell` etc, and thus using HEAD as heuristic was
chosen.