git-cvsimport-3 and incremental imports

I've now spent some time looking at git-cvsimport-3.py from jc/cvsimport-upgrade and made some progress in making it pass more of the Git test suite (my work in progress is at [1]).

However, I think there is a fundamental problem with the way it handles incremental imports and I'm hoping someone with more git-fast-import experience can point me in the right direction.

Currently, cvsps-3 never writes a "from ..." line in the first commit it outputs for a branch, even when the output is restricted by date (i.e. a continuation of a previously imported branch), which results in failure to update branches since git-fast-import is run without "--force". If I make a simple modification so that it does this, it can end up outputting an empty commit (a duplicate of the current tip commit on the branch).

Given that the start date for the import is currently just read from HEAD there is probably scope for this being worse on other branches if they have more recent commits than the current branch.

I don't think there is any way to solve this without giving cvsps more information, probably the last commit time for all git branches, but perhaps I'm missing a fast-import feature that can help solve this problem.

[1] https://github.com/johnkeeping/git/tree/cvsimport-3

John