cvsimport: setup indexes correctly for ancestors and incremental imports

cvsimport: setup indexes correctly for ancestors and incremental imports

Two bugs had slipped in the "keep one index per branch during import"
patch. Both incremental imports and new branches would see an
empty tree for their initial commit. Now we cover all the relevant
cases, checking whether we actually need to setup the index before
preparing the actual commit, and doing it.

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