fetchgit: pass tag directly and make overridable

    constructDrv = stdenvNoCC.mkDerivation;

    excludeDrvArgNames = [

      # Passed via `passthru`

      "tag"

      # Additional stdenv.mkDerivation arguments from derived fetchers.

      "derivationArgs"

          rev ? null,

          name ? urlToName {

            inherit url;

            rev = lib.revOrTag rev tag;

            rev = lib.revOrTag rev finalAttrs.tag;

            # when rootDir is specified, avoid invalidating the result when rev changes

            append = if rootDir != "" then "-${lib.strings.sanitizeDerivationName rootDir}" else "";

          },

              rootDir

              gitConfigFile

              ;

            rev = getRevWithTag { inherit tag rev; };

            inherit tag;

            rev = getRevWithTag {

              inherit (finalAttrs) tag;

              inherit rev;

            };

            postHook =

              if netrcPhase == null then

            passthru = {

              gitRepoUrl = url;

              inherit tag;

            }

            // passthru;

          }