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
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
Only fail adding objects so many times before aborting
Store filenode to blob IDs in map file (again)