RFC: default to indexed HEAD rather than latest HEAD

Created by: keegancsmith

For large monorepos indexing can take a few minutes. This leads to users browsing the HEAD commit having a poor experience when the index is out of date (eg symbols tab, search, etc).

Proposal

When we do not specify a revision (ie rev == ""), default to indexed HEAD. If indexed HEAD is not available, fallback to git HEAD. When browsing rev == "HEAD" use git HEAD. We should consider messaging users when browsing indexed HEAD how far behind it is.

We will likely have to update a few places where we hardcode HEAD to instead use the empty string.

wdyt? cc @slimsag @christinaforney @sourcegraph/search @poojaj-tech