Correct regex for 822 value fields that begin with a colon character

Correct regex for 822 value fields that begin with a colon character

Previously, we parsed "colon: :" => ("colon:", "") and "lol: :lol" =>
("lol:", "lol"). This commit changes these parses to ("colon", "") and
("lol", ":lol") respectfully.

Fields starting with colons can happen with values containing CVS
locations. eg:

  Vcs-Cvs: :ext:_anoncvs@anoncvs.mirbsd.org:/cvs contrib/hosted/tg/deb/makefs

(from /srv/ftp.debian.org/queue/new/makefs_20090808-1.dsc)

This is causing a traceback when examining this package as the regex to
strip the keys in read_changes_or_dsc(..) works differently to this one.

Signed-off-by: Chris Lamb <lamby@debian.org>