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/libexpr/primops/fetchGit.cc

@@ -4,6 +4,7 @@
 #include "store-api.hh"
 #include "pathlocks.hh"
 #include "hash.hh"
+#include "tarfile.hh"
 
 #include <sys/time.h>
 
@@ -164,14 +165,16 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
         if (e.errNo != ENOENT) throw;
     }
 
-    // FIXME: should pipe this, or find some better way to extract a
-    // revision.
-    auto tar = runProgram("git", true, { "-C", cacheDir, "archive", gitInfo.rev });
+    auto source = sinkToSource([&](Sink & sink) {
+        RunOptions gitOptions("git", { "-C", cacheDir, "archive", gitInfo.rev });
+        gitOptions.standardOut = &sink;
+        runProgram2(gitOptions);
+    });
 
     Path tmpDir = createTempDir();
     AutoDelete delTmpDir(tmpDir, true);
 
-    runProgram("tar", true, { "x", "-C", tmpDir }, tar);
+    unpackTarfile(*source, tmpDir);
 
     gitInfo.storePath = store->addToStore(name, tmpDir);