Include extra information in mutt version string for developer builds.

Include extra information in mutt version string for developer builds.

When a build is based on an hg clone, include extra information about
the changeset node, distance from a tagged release, and mq applied patch
count.

For example, after this patch is applied my mutt build identifies itself
(in mutt -v and in <show-version>) as:

Mutt 1.5.21+26,mq+22 (7edc2073390d) (2010-09-15)

I have applied 26 changesets applied since 1.5.21 was tagged, 22 of
which are in my mq patch series.  A 1.5.21 release build that is not
mercurial-based would still appear simply as "1.5.21".