Merge branch 'jk/save-getenv-result'

There were many places the code relied on the string returned from
getenv() to be non-volatile, which is not true, that have been
corrected.

* jk/save-getenv-result:
  builtin_diff(): read $GIT_DIFF_OPTS closer to use
  merge-recursive: copy $GITHEAD strings
  init: make a copy of $GIT_DIR string
  config: make a copy of $GIT_CONFIG string
  commit: copy saved getenv() result
  get_super_prefix(): copy getenv() result

builtin/config.c

@@ -599,7 +599,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 	int nongit = !startup_info->have_repository;
 	char *value;
 
-	given_config_source.file = getenv(CONFIG_ENVIRONMENT);
+	given_config_source.file = xstrdup_or_null(getenv(CONFIG_ENVIRONMENT));
 
 	argc = parse_options(argc, argv, prefix, builtin_config_options,
 			     builtin_config_usage,