[PATCH resubmit 0 of 2] Add template tools to replicate `hg export' style

[Paolo Bonzini]

This series of two patches adds two features to the templating
engine so that a custom style can replicate the `hg export' way
of presenting parents.

The problem is that --debug will always show both parents (and
other debug information such as `searching for copies'); omitting
--debug however will never show the immediately preceding revision.

To do this, I added two features, the `first' filter and the
`allparents' item; using {allparents|first} in the no_parents
entry of a style achieves the desired result.

Each of the two patches implements one of these two features and
adds documentation as well as tests for the feature.

The code is the same as in the first version submitted 23 June,
I tweaked a bit the commit message of part 2.

Test results:
# Ran 344 tests, 16 skipped, 0 failed.

 mercurial/cmdutil.py            |  6 +++++
 mercurial/help.py               |  3 ++
 mercurial/templatefilters.py    |  8 ++++++
 tests/test-command-template     | 12 +++++++++-
 tests/test-command-template.out | 46 +++++++++++++++++++++++++++++++++++
 5 files changed, 74 insertions(+), 1 deletion(-)