config: provide access to the current line number

Users are nowadays trained to see message from CLI tools in the form

    <file>:<lno>: …

To be able to give such messages when notifying the user about
configurations in any config file, it is currently only possible to get
the file name (if the value originates from a file to begin with) via
`current_config_name()`. Now it is also possible to query the current line
number for the configuration.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.h

@@ -309,6 +309,7 @@ int git_config_parse_parameter(const char *, config_fn_t fn, void *data);
 enum config_scope current_config_scope(void);
 const char *current_config_origin_type(void);
 const char *current_config_name(void);
+int current_config_line(void);
 
 /**
  * Include Directives