On this wiki, editing `templates/gitbranch.mdwn` causes a really slow
refresh, orders of magnitude slower than a full rebuild: a large number of
pages depend on that template, or link to a page that embeds that template,
and so on.

I suspect that, as with my optimization pass for `album`'s benefit, the
costly thing is evaluating lots of pagespecs. I'm profiling it to see
whether the problem is there are any low-hanging fruit.

Easy to reproduce offline:

* comment out the `exclude` option in `docwiki.setup`
* `/usr/bin/perl -Iblib/lib ikiwiki.in -setup docwiki.setup -rebuild`
* `touch templates/gitbranch.mdwn`
* `/usr/bin/perl -Iblib/lib ikiwiki.in -setup docwiki.setup -refresh`

NYTProf says:

    # spent 279s (237+41.8) within IkiWiki::bestlink which was called 13988949 times, avg 20µs/call:
    # 13150827 times (222s+37.2s) by IkiWiki::PageSpec::match_link at line 2692, avg 20µs/call
    # 829606 times (14.9s+4.51s) by IkiWiki::PageSpec::match_link at line 2687, avg 23µs/call
    ...
    sub bestlink ($$) {

which is about half the execution time (458s on my laptop).

Adding code to log each call to match_backlink indicates that a large part
of the problem is that it evaluates the pagespec
`backlink(plugins/goodstuff)` up to a million times, with various pages and locations.

--[[smcv]]