The attached patch implements the optional generation of the source
.changes file for pdebuild-internal operation mode.
It also implements the signing of all the generated .changes file
(both the binary and source .changes file, if both were generated;
only one of them, if one was generated).
Please note that, the --no-re-sign option is passed to debsign
from the second invocation on, in order to avoid breaking the
checksums of any previously signed .changes file.
See also bug [#906063], which is related to checksum breaking
(I don't know if my patch also fixes that bug, though...).