Allow folks to periodically update the export of a live repo without
re-exporting from the beginning. This is a performance improvement, but
can also be important for collaboration. For example, for sensitivity
reasons, folks might want to export a subset of a repo and update the
export periodically. While this could be done by just re-exporting the
repository anew each time, there is a risk that the paths used to
specify the wanted subset might need to change in the future; making the
user verify that their paths (including globs or regexes) don't also
pick up anything from history that was previously excluded so that they
don't get a divergent history is not very user friendly. Allowing them
to just export stuff that is new since the last export works much better
for them.
Signed-off-by: Elijah Newren <newren@gmail.com>