boost/generic.nix: reference commit-hash in comment

The explanation for the conditional introduced by
61d9f201baeef4c4bb91ad8a8f5f89b747e0dfe4 is longer than a reasonable
inline comment should be.  It directed the reader to use `git blame`,
but that tends to bitrot.  Let's point the user to a specific nixpkgs
git hash.

This commit cannot be squashed into the previous commit, because a
commit cannot mention its own commit-hash (without performaing an
expensive double-sha1 preimage attack, of course).

pkgs/development/libraries/boost/generic.nix

@@ -82,7 +82,7 @@ let
 
     # TODO: make this unconditional
   ] ++ optionals (stdenv.hostPlatform != stdenv.buildPlatform ||
-                  # required; see this line's `git blame` for an explanation
+                  # required on mips; see 61d9f201baeef4c4bb91ad8a8f5f89b747e0dfe4
                   (stdenv.hostPlatform.isMips && versionAtLeast version "1.79")) [
     "address-model=${toString stdenv.hostPlatform.parsed.cpu.bits}"
     "architecture=${if stdenv.hostPlatform.isMips64