feat(revset): add + as shortcut for "only child"

[claytonrcarter]

This adds + as a postfix revset operator that evaluates to sole(children(...)). I chose + because it matches the similar operator in jj revsets, but I didn't add - for symmetry because I didn't feel the need since we already have ~ and ^.

Note that I chose sole(...) because I have found that I (almost?) always wanted this shorthand in cases where there is only 1 child, and if I wanted all children, I could reach for the regular children(...) function. I don't think I care about this that strongly, though, and I'd be OK to remove sole(...) if preferred.

Handy use cases:

• when it's easier to paste the commit already on the clipboard and just add + or ++ instead of copying the commit you really want :)
• unstacking a stack of branches: git move -s prev-branch+ -d main
• another way to do git next: git sw @+

[arxanas]

Sorry, haven't had time to go through my pending PRs yet. Generally speaking, I would want to be consistent with jj's revset language where possible, since git-branchless is probably a gateway to jj for many users.

[claytonrcarter]

consistent with jj's revset

OK, for now I have reimplemented this to use ! instead of +. Not only does it avoid the conflict with jj, but I found that my original implementation wasn't sophisticated at all enough to correctly handle a case like @+ + @++: it was parsed as (@+ + @+)+ instead of my intended @+ + (@++). I didn't understand the lalrpop grammar enough to fix that. Using ! sidesteps both of these issues, but at the cost of introducing a totally new operator.

If I can get my head around the union conflict and the lalrpop grammar, I would prefer to implement it to mimic jj, so I'll just draft this ! version for now.