Extract filepair_cb from ui-patch.c

Move filepair_cb() from ui-patch.c to ui-shared.c and rename it to
filepair_cb_raw(). This callback will be used in ui-diff.c in a
follow-up patch.

Note that it is not straightforward to extract filepair_cb() from
ui-diff.c which is why it is not done here as well.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-shared.h

@@ -67,4 +67,5 @@ extern void cgit_print_snapshot_links(const char *repo, const char *head,
 				      const char *hex, int snapshots);
 extern void cgit_add_hidden_formfields(int incl_head, int incl_search,
 				       const char *page);
+extern void filepair_cb_raw(struct diff_filepair *pair);
 #endif /* UI_SHARED_H */