Aliases are loaded at WorkspaceCommandHelper::new() as it's easier to warn
invalid declarations there. Not all commands use revsets, but many do, so
I think it's okay to always pay the loading cost. Parsing the declaration
part (i.e. a symbol) should be fast anyway.
The nested error message isn't super readable, but seems good enough.
Config syntax to bikeshed:
- naming: [revset-alias] vs [revset-aliases] ?
- function alias will need quotes: 'f(x)' = 'x'
I'm thinking of adding alias expansion at this stage, and it would be a bit
tedious to pass around mutable context by function parameter. So let's reduce
the number of the intermediate functions.
This also produces a better error message.
