version: drop support for Mercurial 1.9.3

Upcoming patches will clean up some code that makes hg-git work with Mercurial
versions < 2.0.
extsetup: drop support for Mercurial < 1.7
nodetags: drop support for Mercurial < 1.6
getremotechanges: drop support for Mercurial < 1.7
gitrepo: drop support for Mercurial < 1.7
hgrepo.push: drop support for Mercurial < 1.6
hgrepo.tags: drop support for Mercurial < 2.0

A new property called _tagscache was introduced in Mercurial 2.0, so the cache
wasn't actually working.

The contract for tags() also changed at some point -- it stopped returning
nodes that weren't in the repo. This will need to be accounted for if we
start using the tags cache again. However, it isn't very clear whether the
Mercurial tags cache is actually worth doing, since we already have a
separate in-memory cache for Git tags in the handler.
git_handler.save_map: drop support for Mercurial < 1.9
git_handler.save_tags: drop support for Mercurial < 1.9
git_handler: remove old and bogus code for deleting entries from tags cache

This code never worked for Mercurial >= 2.0, since it neither had repo._tags
nor repo.tagscache.
overlay: drop support for Mercurial < 1.9
git_handler: replace util.progress with ui.progress

util.progress was a shim for Mercurial < 1.4.
util: drop support for Mercurial < 1.4