Export Git objects from incremental Mercurial changes

This replaces the brute force Mercurial to Git export with one that is
incremental. It results in a decent performance win and paves the road
for parallel export via using multiple incremental exporters.
Implement parallel Mercurial export to Git trees
squash
Add command-line arguments for gexport
Add command to prune Git mapping of specified HG changesets
Add ability to prune non-changesets from mapping file
Proper batch sizes when number of exports is really small
Only fail adding objects so many times before aborting
Store filenode to blob IDs in map file (again)