Show when we're unpacking an archive into the Git cache

This happens in parallel with the download (which starts later), so
you only see this message when the download has finished but the
import hasn't.

src/libfetchers/github.cc

@@ -254,10 +254,15 @@ struct GitArchiveInputScheme : InputScheme
             getFileTransfer()->download(std::move(req), sink);
         });
 
+        auto act = std::make_unique<Activity>(*logger, lvlInfo, actUnknown,
+            fmt("unpacking '%s' into the Git cache", input.to_string()));
+
         TarArchive archive { *source };
         auto parseSink = getTarballCache()->getFileSystemObjectSink();
         auto lastModified = unpackTarfileToSink(archive, *parseSink);
 
+        act.reset();
+
         TarballInfo tarballInfo {
             .treeHash = parseSink->sync(),
             .lastModified = lastModified

src/libfetchers/tarball.cc

@@ -143,6 +143,9 @@ DownloadTarballResult downloadTarball(
 
     // TODO: fall back to cached value if download fails.
 
+    auto act = std::make_unique<Activity>(*logger, lvlInfo, actUnknown,
+        fmt("unpacking '%s' into the Git cache", url));
+
     AutoDelete cleanupTemp;
 
     /* Note: if the download is cached, `importTarball()` will receive
@@ -167,6 +170,8 @@ DownloadTarballResult downloadTarball(
     auto parseSink = getTarballCache()->getFileSystemObjectSink();
     auto lastModified = unpackTarfileToSink(archive, *parseSink);
 
+    act.reset();
+
     auto res(_res->lock());
 
     Attrs infoAttrs;