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.)
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ echo '[extensions]' >> $HGRCPATH
$ echo "gpg =" >> $HGRCPATH
$ echo "hgext.gpg =" >> $HGRCPATH
$ echo "badext = $abspath" >> $HGRCPATH
$ echo "badext2 =" >> $HGRCPATH
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
hg help [-ec] [TOPIC]
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