Added tag 0.4.0 for changeset a3c3b8077cbe
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.
overlaymanifest: add iteritems(), used by recent hg versions
Only export new Git trees
Add command to prune Git mapping of specified HG changesets
Implement parallel Mercurial export to Git trees
Add command-line arguments for gexport
Only fail adding objects so many times before aborting
Store filenode to blob IDs in map file (again)