git-cvsimport-3: add a version check for new features

Instead of failing with an obscure cvsps error if a feature that is
required is not supported, add a way to check for a given version before
invoking cvsps.

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