revert(3p/git): Revert merge of git upstream at v2.26.2

This causes cgit to serve error pages, which is undesirable.

This reverts commit 5229c9b232, reversing
changes made to f2b211131f.

third_party/git/transport.h

@@ -208,9 +208,6 @@ void transport_check_allowed(const char *type);
 /* Filter objects for partial clone and fetch */
 #define TRANS_OPT_LIST_OBJECTS_FILTER "filter"
 
-/* Request atomic (all-or-nothing) updates when pushing */
-#define TRANS_OPT_ATOMIC "atomic"
-
 /**
  * Returns 0 if the option was used, non-zero otherwise. Prints a
  * message to stderr if the option is not used.