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>

environment.c

@@ -33,6 +33,7 @@ size_t delta_base_cache_limit = 16 * 1024 * 1024;
 char *pager_program;
 int pager_in_use;
 int pager_use_color = 1;
+char *editor_program;
 int auto_crlf = 0;	/* 1: both ways, -1: only when adding git objects */
 
 static const char *git_dir;