Implemented 'tag' command in fast-import.

Implemented 'tag' command in fast-import.

Tags received from the frontend are generated in memory in a simple
linked list in the order that the tag commands were sent by the
frontend.  If multiple different tag objects for the same tag name
get generated the last one sent by the frontend will be the one
that gets written out at termination.  Multiple tag objects for
the same name will cause all older tags of the same name to be lost.

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