GC puts all commits reachable from heads and tags into the GC pack,
and commits reachable only from other refs (e.g. refs/changes) into
GC_REST. The commit-graph contains all commits in GC and GC_REST. This
produces too big commit graphs in some repos, beating the purpose of
loading the index.
Limit the commit graph to commits reachable from heads and tags
(i.e. commits in the GC pack).
Change-Id: I4962faea5a726d2ea3e548af0aeae370a6cc8588
6f73336939