git-cvsimport-3 and incremental imports

[John Keeping <john@keeping.me.uk>]

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