In the "Version Information" panel, try to include branchpoints

Summary:
Fixes T12040. In T12039, a user running local patches followed the report instructions as far as grabbing version information, but didn't update or revert their local changes or try against a clean install before reporting.

This obviously isn't ideal for us, but it's understandable (grabbing version information is much easier than upgrading/reverting), and we can do better about making this information useful: when compiling version information, try to figure out the branchpoint from a known upstream master branch by listing remotes, then running git merge-base against them.

Additionally, explicitly document that we want upstream hashes. We have to have a fallback case in this document anyway (for when you can't get to Config) so hopefully this makes it more likely that we get useful information in initial reports.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12040

Differential Revision: https://secure.phabricator.com/D17103