dpkg-source: fix path to gitshallow file in source format "3.0 (git)"

It was looking in the current directory, which works most of the time,
but not always.

scripts/Dpkg/Source/Package/V3/git.pm

@@ -225,7 +225,7 @@ sub do_extract {
         # Move shallow info file into place, so git does not
         # try to follow parents of shallow refs.
         info(_g("setting up shallow clone"));
-        system("cp", "-f",  $shallow, "$newdirectory/.git/shallow");
+        system("cp", "-f",  $dscdir.$shallow, "$newdirectory/.git/shallow");
         $? && subprocerr("cp");
     }