summary/revert: make --heads DWIM and add alternative

summary/revert: make --heads DWIM and add alternative

When the TopGit branches are not up-to-date, then using
`git merge-base --independent` to discover what the roots
of the TopGit dependency graph may not work properly.

To get the correct TopGit dependency graph roots, examine
the various .topdeps files instead.  The roots are simply
the TopGit branches that are not mentioned in any of the
.topdeps files.

For `tg summary`, perform the TopGit heads computation
for the `--heads` option as that's what's usually intended
there, but add a new `--heads-independent` option to get
the old `git merge-base --independent` variation as that
can sometimes be useful.

For `tg revert`, however, keep the `git merge-base --independent`
behavior for `--heads` as that's supposed to show independent
heads stored in the tag data.  But add a new `--topgit-heads`
that shows only the roots of the TopGit dependency tree that
are stored in the tag data.  Do, however, make the `--rdeps`
option default to using the new `--topgit-heads` mode since,
in that case, that's likely the expected default.

For usability though `tg revert` will accept --heads-independent
and `tg summary` will accept --topgit-heads too.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>