Better merge tool integration

[Eric Siegerman]

On Wed, 2010-06-30 at 20:59 -0400, Gordon Tyler wrote:
> The reason it has an availability check is that I want to provide a
> default set of tools in the configuration that might be available on
the
> path, 

How about the merge tool's version number?  Different versions of
a given tool might need (or at least benefit from) different
configurations, and it could be helpful to include more than one
of those in the default set.

I'm imagining another API call provided by the mergetools module,
to return a tool's version number, determining it in some
tool-dependent way that's stored as part of the per-tool
configuration:  "${tool} --version", "${tool} /V" or whatever.

  - Eric