Return "calculated" checksum if requested w/caching

If a file is downloaded via librepo (e.g. `dnf install --downloadonly`)
then a request to get the checksum via `lr_checksum_fd_compare()` will
not work. It'll only return whether the checksum is valid, and not the
actual checksum. This is the simple fix.

Addresses #233

librepo/checksum.c

@@ -248,6 +248,8 @@ lr_checksum_fd_compare(LrChecksumType type,
                     g_debug("%s: Using checksum cached in xattr: [%s] %s",
                             __func__, checksum_key, buf);
                     *matches = (strcmp(expected, buf) == 0);
+                    if (calculated)
+                      *calculated = g_strdup(checksum);
                     return TRUE;
                 }
             } else {