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.)
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present