gitserver: check clone in progress if not cloned

Created by: keegancsmith

When we run a git command we first check if there is a clone in progress for the repo. If there is we return saying the repo is busy cloning. However, the clone may in fact exist since our cloning logic clones to a temp directory and renames into place. For example our reclone script.

This commit instead will use the existing clone if it is there. It does this by only checking the clone lock if the repo is not in place.

Part of https://github.com/sourcegraph/sourcegraph/issues/10274