lib/sources.nix@commitIdFromGitRepo: parenthesize what should always have been

Fixes error: cannot coerce a partially applied built-in function to a string, at lib/sources.nix:59:32

I don't understand how this used to work, but doesn't work anymore?

lib/sources.nix

···
            else if lib.pathExists packedRefsName
            then
              let fileContent = readFile packedRefsName;
-                 matchRef    = match ".*\n([^\n ]*) " + file + "\n.*" fileContent;
+                 matchRef    = match (".*\n([^\n ]*) " + file + "\n.*") fileContent;
              in if   isNull matchRef
                 then throw ("Could not find " + file + " in " + packedRefsName)
                 else lib.head matchRef