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>

builtin-config.c

@@ -178,14 +178,14 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 			char *home = getenv("HOME");
 			if (home) {
 				char *user_config = xstrdup(mkpath("%s/.gitconfig", home));
-				setenv("GIT_CONFIG", user_config, 1);
+				setenv(CONFIG_ENVIRONMENT, user_config, 1);
 				free(user_config);
 			} else {
 				die("$HOME not set");
 			}
 		}
 		else if (!strcmp(argv[1], "--system"))
-			setenv("GIT_CONFIG", ETC_GITCONFIG, 1);
+			setenv(CONFIG_ENVIRONMENT, ETC_GITCONFIG, 1);
 		else if (!strcmp(argv[1], "--null") || !strcmp(argv[1], "-z")) {
 			term = '\0';
 			delim = '\n';

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;
 	}