Add explanatory comment for transport-helpers refs mapping.

Add explanatory comment for transport-helpers refs mapping.

The patch below adds a comment to fetch_with_import() explaining the
loop that saves the fetched commit names after 'git fast-import' has
done its work.  It avoids some confusion about which refs the
fast-import stream is supposed to use to write its result.

Signed-off-by: Florian Achleitner <florian.achleitner.2.6.31@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>