FR: support string concatenation in the revset language

[afranchuk]

Is your feature request related to a problem? Please describe.
Revset aliases are very useful, however I've found that I can't concatenate strings, which would make them even more useful. In particular, I'm trying to write an alias which matches changes with a particular form of string in the subject line (that combines a fixed string with the alias parameter).

Describe the solution you'd like
Something like concat() or ++ (like the templating language) would be great.

Describe alternatives you've considered
I've tried to create an equivalent alias combining multiple separate subject() matches, but it's not able to be as accurate as one with the exact string that needs to be matched.

[yuja]

This might be a pre-processing (or alias-specific) syntax.