R1:75781dba1add

Improve autocomplete behavior in lists and with noncompleting results

Summary:
Ref T10163. Currently, we don't activate on indented lines, but were too aggressive about this, and would not activate on lines like ` - Hey, @user...`, where we should.

Instead, don't activate on indented lines if there's only an indent (i.e., `#` probably means enumerated list).

Also, if results don't have autocompletes (rare but possible with projects missing slugs), improve behavior.

Test Plan:
- Typed ` #a`, got no…