Default `fg` when only one job is backgrounded

[atagen]

Describe the problem: When only one job is in the background, it seems excessive to require the PID/FID to reactivate it - the user can only really mean one thing by typing fg.

Possible ways to implement: Automatically foreground process on fg when there is only a single job.

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[lmorg]

Coincidentally, I've been thinking about adding a few enhancements to fg too. Such as supporting % prefixes like Bash (et al) do.

So it wouldn't take any extra effort to make fg default to %1 if no parameters are supplied.

[atagen]

Nice! Perhaps a touch bikesheddy but IMO Bash-esque % prefixes can induce extra cognitive load when you're juggling more than 1-2 processes. If you're already going that route, would you consider perhaps %partialprocessname, ie. fg %py could foreground the process called by python my_web_server.py? (Of course, preferring errors to ambiguity, so if there are two bg python processes we don't act.)

[lmorg]

I could easily add support for partial process names without the % prefix. Basically the logic would go:

1. is it an integer? If so, it's a FID
2. is it a % prefixed integer? If so, follow Bash's format (for people with Bash muscle memory)
3. otherwise, assume its a partial process name