Make get_valid_git_username_email static

Also alias where it is used to make code a little easier to read.
Implement TreeTracker for incremental tree calculation

This class makes exporting Mercurial changesets to Git much faster.
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
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)