Removed ancient and unused code to find the last imported revision from previous imports
to use for the current import by looking at the p4 tags. The current approach of using
the log message works better.
Always pass a sha1 for the initial parent so that git-fast-import doesn't think
it's creating a new branch from itself. It's a sensible error in general but
in the case of incremental imports we have to apply force :)
Reduce the number of false "merges" by skipping "branch from" entries in the integrated output as well as by ignoring integrations of future (newer) changes.