cvsimport: keep one index per branch during import

With this patch we have a speedup and much lower IO when
importing trees with many branches. Instead of forcing
index re-population for each branch switch, we keep
many index files around, one per branch.

Signed-off-by: Martin Langhoff <martin@catalyst.net.nz>
Signed-off-by: Junio C Hamano <junkio@cox.net>