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/attrs.cc

@@ -104,4 +104,9 @@ std::map<std::string, std::string> attrsToQuery(const Attrs & attrs)
     return query;
 }
 
+Hash getRevAttr(const Attrs & attrs, const std::string & name)
+{
+    return Hash::parseAny(getStrAttr(attrs, name), htSHA1);
+}
+
 }