Support Perforce changelists in src code-intel upload

[varungandhi-src]

Right now, there is a -commit flag, but if you try to pass a Perforce changelist, but the backend validates that the value is a Git hash.

unexpected status code: 400 (failed to enqueue payload: commit must be a 40-character revhash)

We should perhaps keep using the same flag, but have it allow passing a changelist instead.

In terms of design, perhaps we should require a cl:<blah> format (or similar) instead of accepting a bare identifier and then just assume that it is a changelist if it is not a 40-character hash. That way, we can add support for other formats later without having to have complicated inference logic.

[mike-r-mclaughlin]

Requested by https://github.com/sourcegraph/accounts/issues/6938