Implement new ** glob semantics per Git

[okdana]

Hey! Looks like the new ** glob semantics discussed on the Git mailing list have been merged:

git/git@627186d

(Just to reiterate, the change makes a ** used in any context other than **/, /**/, or /** act as two regular *s.)

I believe this change will appear not in the up-coming release (2.20), but the one right after that.

I'd been playing with globset in preparation for this landing, which is how i discovered the issues i mentioned in #1093, but i'm not happy with my changes, so i don't have a PR yet. I was also unsure whether you'd want to wait until this actually appears in a released Git or what. But i wanted to make a note of it, anyway, just to track it.

[okdana]

I was wrong about when this would come out. It's in 2.20, released today: http://lkml.iu.edu/hypermail/linux/kernel/1812.1/00293.html

And the updated documentation is live: https://git-scm.com/docs/gitignore

The difference in behaviour can be confirmed with e.g. git ls-files in the ripgrep repo:

% ~cellar/git/2.19.2/bin/git ls-files --ignored -x '/**.md'
# (no output)
% ~cellar/git/2.20.0/bin/git ls-files --ignored -x '/**.md'
CHANGELOG.md
FAQ.md
GUIDE.md
ISSUE_TEMPLATE.md
README.md

(We'd already established this, but just to be clear, Git did actually accept ** in many cases where it said it didn't, so some documented-illegal patterns, like [A-Z]**.md, won't demonstrate any difference)

[BurntSushi]

Awesome! I'm happy to see the ** issue finally moving towards a solid answer. :-)

Sorry I haven't addressed this sooner or your PR yet. Everything looks good, and thanks for all the legwork on this. From what I've skimmed, everything sounds right, but I haven't quite gotten the time to sink my teeth into the details yet.