gitattributes: drop support for GIT_ATTR_NOGLOBAL

test-lib sets $HOME to protect against pollution from user settings,
so setting GIT_ATTR_NOGLOBAL would be redundant.  Simplify by
eliminating support for that environment variable altogether.

GIT_ATTR_NOGLOBAL was introduced in v1.7.4-rc0~208^2 (Add global and
system-wide gitattributes, 2010-09-01) as an undocumented feature for
use by the test suite.  It never ended up being used (neither within
git.git nor in other projects).

This patch does not affect GIT_ATTR_NOSYSTEM, which should still be
useful.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -478,11 +478,6 @@ int git_attr_system(void)
 	return !git_env_bool("GIT_ATTR_NOSYSTEM", 0);
 }
 
-int git_attr_global(void)
-{
-	return !git_env_bool("GIT_ATTR_NOGLOBAL", 0);
-}
-
 static int git_attr_config(const char *var, const char *value, void *dummy)
 {
 	if (!strcmp(var, "core.attributesfile"))
@@ -511,7 +506,7 @@ static void bootstrap_attr_stack(void)
 		}
 
 		git_config(git_attr_config, NULL);
-		if (git_attr_global() && attributes_file) {
+		if (attributes_file) {
 			elem = read_attr_from_file(attributes_file, 1);
 			if (elem) {
 				elem->origin = NULL;