git-cvsimport-3: pass "-i" to cvsps for incremental imports

This flag is required for cvsps to output the correct incremental
fast-import stream, so git-cvsimport-3 will fail if the available cvsps
does not support it.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: remove an unused variable

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: relax Git directory creation rules

Unlike git-cvsimport-2, git-cvsimport-3 does not allow a Git repository
to be created in an existing CVS checkout.  Change this by explicitly
setting the Git directory and deciding what to do based on whether that
exists.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: don't leak file handles

Use the "with" statement when opening files to ensure that they are
closed at the correct time.

Signed-off-by: John Keeping <john@keeping.me.uk>
git-cvsimport-3: use a file to record the import timestamp

This removes errors when the tip of the checked out branch is not the
most recent commit imported from CVS.

Calculate the import time by taking the most recent change that is on a
branch which changed during the import.

Signed-off-by: John Keeping <john@keeping.me.uk>