new_filter: determine extra_args from filter type

Currently the number of extra arguments is linked hard to the type of
the filter. This is also logical since it would be confusing to have
a different number of arguments for the same type of filter depending
on the context under which the filter is run (unless ofcourse one the
parameters would make the context clear, which is currently not the
case).

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -51,6 +51,10 @@ typedef void (*configfn)(const char *name, const char *value);
 typedef void (*filepair_fn)(struct diff_filepair *pair);
 typedef void (*linediff_fn)(char *line, int len);
 
+typedef enum {
+	ABOUT, COMMIT, SOURCE
+} filter_type;
+
 struct cgit_filter {
 	char *cmd;
 	char **argv;