GitDownloadStrategy optimization

The current series of fetch invocations in GitDownloadStrategy has the
unfortunate behavior of fetching full history even in shallow clones
that only need the history between the clone point and the remote HEAD.
It should be possible to determine if it is actually necessary to fetch
the full history, including all tags, and if it is not to avoid this
overhead.

Fixes Homebrew/homebrew#11958, and several other recurring problems.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>