phases: rename _phasemaxrev to _loadedrevslen to clarify it isn't max value "maxrev" sounds like max(0:tip), but it is actually len(0:tip).

lfs: add an experimental config to override User-Agent for the blob transfer This will allow developers to test against various server implementations. I didn't put it under [devel] because it's possible that some user needs to use it in the field.

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