Drop remaining uses of external "tar"

Also, fetchGit now runs in O(1) memory since we pipe the output of
'git archive' directly into unpackTarball() (rather than first reading
it all into memory).

src/nix-prefetch-url/nix-prefetch-url.cc

@@ -9,6 +9,7 @@
 #include "legacy.hh"
 #include "finally.hh"
 #include "progress-bar.hh"
+#include "tarfile.hh"
 
 #include <iostream>
 
@@ -192,8 +193,7 @@ static int _main(int argc, char * * argv)
                 if (hasSuffix(baseNameOf(uri), ".zip"))
                     runProgram("unzip", true, {"-qq", tmpFile, "-d", unpacked});
                 else
-                    // FIXME: this requires GNU tar for decompression.
-                    runProgram("tar", true, {"xf", tmpFile, "-C", unpacked});
+                    unpackTarfile(tmpFile, unpacked, baseNameOf(uri));
 
                 /* If the archive unpacks to a single file/directory, then use
                    that as the top-level. */