tests/test-template-engine.t

tags: introduce _readtaghist function

The existing _readtags function has been modified a little and renamed
_readtaghist. A new _readtaghist function has been added, which is a wrappger
around _readtaghist. Its output is the same as the old _readtaghist.

The purpose of this change is to make it possible to automatically merge tag
files. In order to do so we will need to get the line numbers for each of the
tag-node pairs on the first merge parent.

This is not used yet, but will be used on a follow up patch that will introduce
an automatic tag merge algorithm.

I performed some tests to compare the effect of this change. I used timeit to
run the test-tags.t test a 9 times with and without this patch. The results
were:

- without this patch: 3 loops, best of 3: 8.55 sec per loop
- with this patch:    3 loops, best of 3: 8.49 sec per loop

The the test was on average was slightly faster with this patch (although the
difference was probably not statistically significant).

  $ cat > engine.py << EOF
  > 
  > from mercurial import templater
  > 
  > class mytemplater(object):
  >     def __init__(self, loader, filters, defaults):
  >         self.loader = loader
  > 
  >     def process(self, t, map):
  >         tmpl = self.loader(t)
  >         for k, v in map.iteritems():
  >             if k in ('templ', 'ctx', 'repo', 'revcache', 'cache'):
  >                 continue
  >             if hasattr(v, '__call__'):
  >                 v = v(**map)
  >             v = templater.stringify(v)
  >             tmpl = tmpl.replace('{{%s}}' % k, v)
  >         yield tmpl
  > 
  > templater.engines['my'] = mytemplater
  > EOF
  $ hg init test
  $ echo '[extensions]' > test/.hg/hgrc
  $ echo "engine = `pwd`/engine.py" >> test/.hg/hgrc
  $ cd test
  $ cat > mymap << EOF
  > changeset = my:changeset.txt
  > EOF
  $ cat > changeset.txt << EOF
  > {{rev}} {{node}} {{author}}
  > EOF
  $ hg ci -Ama
  adding changeset.txt
  adding mymap
  $ hg log --style=./mymap
  0 97e5f848f0936960273bbf75be6388cd0350a32b test

  $ cat > changeset.txt << EOF
  > {{p1rev}} {{p1node}} {{p2rev}} {{p2node}}
  > EOF
  $ hg ci -Ama
  $ hg log --style=./mymap
  0 97e5f848f0936960273bbf75be6388cd0350a32b -1 0000000000000000000000000000000000000000
  -1 0000000000000000000000000000000000000000 -1 0000000000000000000000000000000000000000

  $ cd ..