Support creation of merge commits in fast-import.

Support creation of merge commits in fast-import.

Some importers are able to determine when branch merges occurred
within their source data.  In these cases they will want to supply
the correct commits to fast-import so that a proper merge commit
will exist in Git.  This is now supported by supplying a 'merge '
command after the commit message and optional from command.

A merge is not actually performed by fast-import, its assumed that
the frontend performed any sort of merging activity already and
that fast-import should simply be storing its result.

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