git_handler: return filtered refs as an OrderedDict

This actually has no direct impact here, but it allows extensions to customize
the list of filtered refs and be sure of the order in which they'll be
processed.
git_handler.filter_refs: sort returned refs if none are provided

Again, no direct impact in stock hg-git, but super useful for extensions.
git_handler: support wildcards for Git branch name matching

This is quite similar to syntax Git supports. In the future maybe core
Mercurial could be extended to support this, but I think this independently
makes sense in hg-git.
git_handler: introduce a function and config to filter by minimum date

This is useful if there's no desire to import old Git branches, but new ones
should be.
pull: support git.mindate
gimport: support git.mindate
readme: add docs for git.mindate
tests: fix file modes