Presize all StringBuilders to 64 chars. The default for StringBuilder is
16, but pretty much all refs are longer than this, e.g.
"refs/changes/01/1" is already 17. The longest ref in this file
currently is cache-automerge at 62, followed by robot-comments at 32+;
64 is enough for now.
Add a variant of shard that takes a StringBuilder, since most places
within this file already have their own.
Change-Id: I7050620ca55266c8cff179902d564ce118c12806
aa75aac57d