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.c

@@ -3338,6 +3338,14 @@ enum config_scope current_config_scope(void)
 		return current_parsing_scope;
 }
 
+int current_config_line(void)
+{
+	if (current_config_kvi)
+		return current_config_kvi->linenr;
+	else
+		return cf->linenr;
+}
+
 int lookup_config(const char **mapping, int nr_mapping, const char *var)
 {
 	int i;