Do not have .pc in dgit's git history for format `3.0 (quilt)'

packages.

 * When constructing a git tree from a .dsc (which we do by unpacking
   it in a temporary directory), drop any .pc directory.

 * Rewrite the quilt fixup preparation: we now fake up a .dsc so that
   we can regenerate the .pc, which is needed by dpkg-source.

 * If after we have committed any quilt fixup, there is a .pc
   directory, make a commit removing it.

 * We now construct new quilt patches in a temporary directory, so
   there is no problem about ignored files or --ignore-dirty.  Remove
   the mention of this limitation from the manpage.

 * Be less critical of `3.0 (quilt)' and improve the documentation
   relating to the interaction between dpkg-source and dgit.  Mention
   git-dpm.

 * When `committing quilty patches', do not re-add a deleted .pc
   directory (while still permitting updates to an existing .pc -
   although the .pc will then disappear in the next commit).

 * In the test suite, disregard differences found in .pc.

 * cmd_quilt_fixup sets $package for the benefit of
   build_maybe_quilt_fixup