Print out the edge commits for each packfile in fast-import.

Print out the edge commits for each packfile in fast-import.

To help callers repack very large repositories into a series of
packfiles fast-import now outputs the last commits/tags it wrote to
a packfile when it prints out the packfile name.  This information
can be feed to pack-objects --revs to repack.  For the first pack
of an initial import this is pretty easy (just feed those SHA1s on
stdin) but for subsequent packs you want to feed the subsequent
pack's final SHA1s but also all prior pack's SHA1s prefixed with
the negation operator.  This way the prior pack's data does not
get included into the subsequent pack.

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