Summary:
This commit takes advantage of git 2.5.0 being able to fetch a
requested revision rather than relying on the desired revision being within the
depth limited fetch.

This relies on having git 2.5.0 on the server which is true for all
of the projects we have manifests for; this shows zero matches:

```
$ rg repo_url opensource/fbcode_builder/manifests | grep -v github
```

We've had a couple of situations recently where folks have run into issues with
the commit rate in folly being higher than then fetch depth, so this should
address that.

Refs: https://github.com/facebook/watchman/issues/866

Reviewed By: fanzeyi

Differential Revision: D24747992

fbshipit-source-id: e9b67c61dddc9f55e05d8984e8d210e7d2faabcb