CONTRIBUTING

lfs: add git to the User-Agent header for blob transfers

As we were trying to transition off of the non production lfs-test-server for
further experimenting, one of the problems we ran into was interoperability.  A
coworker setup gitbucket[1] to act as the blob server, tested with git, and
passed it off to me.  But push failed with a message saying "abort: LFS server
returns invalid JSON:", and then proceeded to dump a huge HTML page to the
screen.  It turns out that it is assuming that git is the only thing that wants
to do a blob transfer, and everything else is a web browser wanting HTML.

It's only a single data point, but I suspect other things may be doing this too.
RFC7231 gives an example [2] of listing multiple products in decreasing order of
significance.  Since the standard provides for this, and since it works with the
one problematic server I found, I'm just enabling this by default for a better
UX.

There's nothing significant about the version of git chosen, other than it is
the current version.

[1] https://github.com/gitbucket/gitbucket/
[2] https://tools.ietf.org/html/rfc7231#page-46

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