FR: Allow creating default flags per command

[matts1]

Is your feature request related to a problem? Please describe.
I often want to set a default set of options to use for a command. For example:

• rebase --skip-empty
• log --no-pager (I like paging for diffs but not for logs)

This can currently be achieved via an alias, but you cannot alias an existing command to something (eg. log = ["log", "--no-pager"]).

Describe the solution you'd like

[command-defaults.log]
pager = false

[command-defaults.rebase]
skip_empty = true

We'd then simply modify clap to use defaults from a function instead of a static value (which appears to be possible - clap-rs/clap#1634 (comment))

Describe alternatives you've considered
Aliasing log to log could work, and might be simpler to implement, but what does the following do:

[aliases]
log = ["log", "--no-pager"]
log-head = ["log", "@"]

In this scenario, it's unclear whether we should use log --no-pager @ or log @.

[martinvonz]

This is a duplicate of the closed #1509. See the explanation there, but feel free to comment there (or here) if you think we missed anything.