tests/test-hghave.t

log: rewrite default template to use labels (issue2866)

This is a complete rewrite of the default template to use labels. This
seems ultimately useless to me in most cases. The biggest benefit of
this patch to me seems to be a fairly complicated example of the
templating engine. It was a lot of hard work to figure out the precise
acceptable syntax, since it's almost undocumented. Hat tip to Steve
Losh's smartlog template, which helped me figure out a lot of the
syntax. Hopefully later I can use the present default log template
as an example for documenting the templating engine.

A test is attached. My goal was to match the --color=debug output,
which may differ slightly in newlines from the actual ANSI escape
codes output. I consider this an acceptable invisible deviation.

There seems to be a considerable slowdown with this rewrite.

Before:

    $ time hg log -T default -r .~100::. > /dev/null
    real  0m0.882s
    user  0m0.812s
    sys   0m0.064s
    $ time hg log -T default -r .~100::. > /dev/null
    real  0m0.872s
    user  0m0.796s
    sys   0m0.068s
    $ time hg log -T default -r .~100::. > /dev/null
    real  0m0.917s
    user  0m0.836s
    sys   0m0.076s

After:

    $ time hg log -T default -r .~100::. > /dev/null
    real  0m1.480s
    user  0m1.392s
    sys   0m0.072s
    $ time hg log -T default -r .~100::. > /dev/null
    real  0m1.500s
    user  0m1.400s
    sys   0m0.088s
    $ time hg log -T default -r .~100::. > /dev/null
    real  0m1.462s
    user  0m1.364s
    sys   0m0.092s

Following the maxim, "make it work, profile, make it faster, in that
order", I deem this slowdown acceptable for now.

I suspect but have not confirmed that a big slowdown comes from
calling keywords twice in the file templates, once to test the
existence of output and again to actually list the output. If so, a
simple speedup might be to improve the templating engine to cache
keywords when called more than once on the same revision.

TODO: I found a bug while working on this. The following stack traces:

    hg log -r . -T '{ifcontains(phase, "secret public", "lol", "omg")}\n'

Testing that hghave does not crash when checking features

  $ "$TESTDIR/hghave" --test-features 2>/dev/null