Teach fast-import how to clear the internal branch content.

Teach fast-import how to clear the internal branch content.

Some frontends may not be able to (easily) keep track of which files
are included in the branch, and which aren't.  Performing this
tracking can be tedious and error prone for the frontend to do,
especially if its foreign data source cannot supply the changed
path list on a per-commit basis.

fast-import now allows a frontend to request that a branch's tree
be wiped clean (reset to the empty tree) at the start of a commit,
allowing the frontend to feed in all paths which belong on the branch.

This is ideal for a tar-file importer frontend, for example, as
the frontend just needs to reformat the tar data stream into a gfi
data stream, which may be something a few Perl regexps can take
care of. :)

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>