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]]

> [[!template id=gitbranch branch=smcv/ready/perf
author="[[Simon McVittie|smcv]]"
browse=http://git.pseudorandom.co.uk/smcv/ikiwiki.git/shortlog/refs/heads/ready/perf]]
> [[!tag patch users/smcv/ready]]
>
> Previously, if a page like `plugins/trail` contained a conditional like
>
>     \[[!if test="backlink(plugins/goodstuff)" all=no]]
>
> (which it gets via `templates/gitbranch`), then the
> [[plugins/conditional]] plugin would give `plugins/trail` a dependency on
> `(backlink(plugins/goodstuff)) and plugins/trail`. This dependency is
> useless: that pagespec can never match any page other than
> `plugins/trail`, but if `plugins/trail` has been modified or deleted,
> then it's going to be rendered or deleted *anyway*, so there's no point
> in spending time evaluating match_backlink for it.
>
> Conversely, the influences from the result were not taken into account,
> so `plugins/trail` did not have the
> `{ "plugins/goodstuff" => $DEPEND_LINKS }` dependency that it should.
>
> We should invert that, depending on the influences but not on the test.
>
> This is at least an order of magnitude faster: when I edit the docwiki
> as described above, a refresh takes 37s with nytprof overhead, compared
> with 458s with nytprof overhead before this change. Without nytprof,
> that refresh takes 14s, which is faster than the 24s rebuild again.
> I didn't record how long the refresh took without nytprof before this
> change, but it was something like 200s.
>
> `bestlink` is still the single most expensive function in this refresh
> at ~ 9.5s, with `match_glob` at ~ 5.2s as the runner-up.
> --[[smcv]]