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>

cache.h

@@ -560,6 +560,8 @@ extern char *pager_program;
 extern int pager_in_use;
 extern int pager_use_color;
 
+extern char *editor_program;
+
 /* base85 */
 int decode_85(char *dst, const char *line, int linelen);
 void encode_85(char *buf, const unsigned char *data, int bytes);