[PATCH 0 of 2] Better support for diffing individual changesets.

[Sune Foldager]

These patches add a -c (--changeset) option to diff and extdiff.

When used (along a single revision given with -r), it behaves a bit
like hg log -p -r, i.e. it returns the diff between that changeset
and its (first) parent. Use --switch-parent to select the other parent.

If no revision is given, --switch-parent can still be used to diff
the working directory against the second parent.