cvsimport: move over to using git-for-each-ref to read refs.

cvsimport: move over to using git-for-each-ref to read refs.

cvsimport opens all of the files in $GIT_DIR/refs/heads and reads
out the sha1's in order to work out what time the last commit on
that branch was made (in CVS) thus allowing incremental updates.
However, this takes no account of hierachical refs naming producing
the following error for each directory in $GIT_DIR/refs:

  Use of uninitialized value in chomp at /usr/bin/git-cvsimport line 503.
  Use of uninitialized value in concatenation (.) or string at
					/usr/bin/git-cvsimport line 505.
  usage: git-cat-file [-t|-s|-e|-p|<type>] <sha1>

Take advantage of the new packed refs work to use the new
for-each-ref iterator to get this information.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>