reenable unlimited pdiff files download

In 51fc6def77edfb1f429a48e5169519e9e05a759b we limited the amount of
pdiff to be downloaded per index to 20. This was a compromise between
not letting it go overboard (becoming even slower) and not using
bandwidth needlessly. Now that with the POC the speed reason is gone it
makes sense again to download as much files as we possible can via pdiff
to save bandwidth (and possibly even time).

It also avoids problems with the limit in cases we were we deal with a
server merged archieve as this limit assumes a strict patch progression.

apt-pkg/acquire-item.cc

@@ -462,7 +462,7 @@ bool pkgAcqDiffIndex::ParseDiffIndex(string IndexDiffFile)		/*{{{*/
 	 if (available_patches.empty() == false)
 	 {
 	    // patching with too many files is rather slow compared to a fast download
-	    unsigned long const fileLimit = _config->FindI("Acquire::PDiffs::FileLimit", 20);
+	    unsigned long const fileLimit = _config->FindI("Acquire::PDiffs::FileLimit", 0);
 	    if (fileLimit != 0 && fileLimit < available_patches.size())
 	    {
 	       if (Debug)