[PATCH] archimport - add merge detection

[PATCH] archimport - add merge detection

We now keep track of the patches merged in each branch since they have
diverged, using the records that the Arch "logs" provide. Merge parents
for a commit are defined if we are merging a series of patches that starts
from the mergebase.

If patches from a related branch are merged out-of-order, we keep track of
how much has been merged sequentially -- the tip of that sequential merge
is our new parent from that branch.

This mechanism works very well for branches that merge in dovetail and/or
flying fish patterns, probably less well for others.

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