Add a Git-based content-addressed tarball cache

GitArchiveInputScheme now streams tarballs into a Git repository. This
deduplicates data a lot, e.g. when you're fetching different revisions
of the Nixpkgs repo. It also warns if the tree hash returned by GitHub
doesn't match the tree hash of the imported tarball.

src/libfetchers/git-utils.hh

@@ -69,6 +69,8 @@ struct GitRepo
         time_t lastModified;
     };
 
+    virtual TarballInfo importTarball(Source & source) = 0;
+
     virtual bool hasObject(const Hash & oid) = 0;
 
     virtual ref<InputAccessor> getAccessor(const Hash & rev) = 0;
@@ -85,6 +87,14 @@ struct GitRepo
     virtual void verifyCommit(
         const Hash & rev,
         const std::vector<fetchers::PublicKey> & publicKeys) = 0;
+
+    /**
+     * Given a Git tree hash, compute the hash of its NAR
+     * serialisation. This is memoised on-disk.
+     */
+    virtual Hash treeHashToNarHash(const Hash & treeHash) = 0;
 };
 
+ref<GitRepo> getTarballCache();
+
 }