getenv/setenv: use constants if available

There were places using "GIT_DIR" instead of GIT_DIR_ENVIRONMENT and
"GIT_CONFIG" instead of CONFIG_ENVIRONMENT.  This makes it easier to
find all places touching an environment variable using git grep or
similar tools.

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -252,7 +252,7 @@ char *enter_repo(char *path, int strict)
 
 	if (access("objects", X_OK) == 0 && access("refs", X_OK) == 0 &&
 	    validate_headref("HEAD") == 0) {
-		setenv("GIT_DIR", ".", 1);
+		setenv(GIT_DIR_ENVIRONMENT, ".", 1);
 		check_repository_format();
 		return path;
 	}