Add a new option 'core.askpass'.

Setting this option has the same effect as setting the environment variable
'GIT_ASKPASS'.

Signed-off-by: Knut Franke <k.franke@science-computing.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -37,6 +37,7 @@ size_t delta_base_cache_limit = 16 * 1024 * 1024;
 const char *pager_program;
 int pager_use_color = 1;
 const char *editor_program;
+const char *askpass_program;
 const char *excludes_file;
 enum auto_crlf auto_crlf = AUTO_CRLF_FALSE;
 int read_replace_refs = 1;