fetchgit: pass rev as revCustom and make overridable

          rev ? null,

          name ? urlToName {

            inherit url;

            rev = lib.revOrTag rev finalAttrs.tag;

            rev = lib.revOrTag finalAttrs.revCustom finalAttrs.tag;

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

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

          },

              gitConfigFile

              ;

            inherit tag;

            revCustom = rev;

            rev = getRevWithTag {

              inherit (finalAttrs) tag;

              inherit rev;

              rev = finalAttrs.revCustom;

            };

            postHook =