allow blob extract blobs by head/path combination

If blob is invoked with no id=, it tries to look up h= and search for path= in
 there. Once found, proceed as normal, otherwise, fail as normal.

Signed-off-by: Michael Krelin <hacker@klever.net>

ui-blob.h

@@ -1,6 +1,6 @@
 #ifndef UI_BLOB_H
 #define UI_BLOB_H
 
-extern void cgit_print_blob(const char *hex, char *path);
+extern void cgit_print_blob(const char *hex, char *path, const char *head);
 
 #endif /* UI_BLOB_H */