Make patch.diff filelog cache LRU of 20 files. Fixes
issue1738.
20 files is as fast as 200 for hg diff -r 28015:30103 of mozilla-central.
Ideally we'd use util.lrucachefunc, but the interface doesn't quite work.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.