matcher: use re2 bindings if available There are two sets of Python re2 bindings available on the internet; this code works with both. Using re2 can greatly improve "hg status" performance when a .hgignore file becomes even modestly complex. Example: "hg status" on a clean tree with 134K files, where "hg debugignore" reports a regexp 4256 bytes in size. no .hgignore: 1.76 sec Python re: 2.79 re2: 1.82 The overhead of regexp matching drops from 1.03 seconds with stock re to 0.06 with re2. (For comparison, a git repo with the same contents and .gitignore file runs "git status -s" in 1.71 seconds, i.e. only slightly faster than hg with re2.)