revsetlang: add a hint for more useful parse errors
This logic is largely based on the similar logic added to template error
messages in D2608 and D2609, but with a few tweaks based on how revsets
actually work.
Differential Revision: https://phab.mercurial-scm.org/D2619
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}