Added branch load counter to fast-import.

Added branch load counter to fast-import.

If the branch load count exceeds the number of branches created then
the frontend is causing fast-import to page branches into and out of
memory due to the way its ordering its commits.  Performance can
likely be increased if the frontend were to alter its commit
sequence such that it stays on one branch before switching to another
branch, then never returns to the prior branch.

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