merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/trace2/tr2_cfg.h

@@ -7,6 +7,12 @@
  */
 void tr2_cfg_list_config_fl(const char *file, int line);
 
+/*
+ * Iterate over all "interesting" environment variables and emit 'def_param'
+ * events for them to TRACE2.
+ */
+void tr2_list_env_vars_fl(const char *file, int line);
+
 /*
  * Emit a "def_param" event for the given key/value pair IF we consider
  * the key to be "interesting".
@@ -16,4 +22,6 @@ void tr2_cfg_set_fl(const char *file, int line, const char *key,
 
 void tr2_cfg_free_patterns(void);
 
+void tr2_cfg_free_env_vars(void);
+
 #endif /* TR2_CFG_H */