launch_editor(): Heed GIT_EDITOR and core.editor settings

In the commit 'Add GIT_EDITOR environment and core.editor
configuration variables', this was done for the shell scripts.
Port it over to builtin-tag's version of launch_editor(), which
is just about to be refactored into editor.c.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -426,6 +426,11 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.editor")) {
+		editor_program = xstrdup(value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }