Simplify is_kept_pack()

This removes --unpacked=<packfile> parameter from the revision parser, and
rewrites its use in git-repack to pass a single --kept-pack-only option
instead.

The new --kept-pack-only option means just that.  When this option is
given, is_kept_pack() that used to say "not on the --unpacked=<packfile>
list" now says "the packfile has corresponding .keep file".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-repack.sh

@@ -68,10 +68,13 @@ case ",$all_into_one," in
 			if [ -e "$PACKDIR/$e.keep" ]; then
 				: keep
 			else
-				args="$args --unpacked=$e.pack"
 				existing="$existing $e"
 			fi
 		done
+		if test -n "$existing"
+		then
+			args="--kept-pack-only"
+		fi
 		if test -n "$args" -a -n "$unpack_unreachable" -a \
 			-n "$remove_redundant"
 		then