string repeat: allow omission of -n

[lavafroth]

Description

string repeat will expect the first positional argument as the count if it is not supplied through the -n flag.

Fixes issue #9332

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[krobelus]

this history looks more complicated than necessary, can you simplify it (remove merge commits, squash commits as appropriate)

[lavafroth]

Sure thing.

[henrikhorluck]

Thank you for your contribution! I found a minor behaviour change that we can avoid, and some minor code style nits, but otherwise LGTM!

This needs relnotes and documentation updates in CHANGELOG.rst, doc_src/cmds/string-repeat.rst and doc_src/cmds/string.rst. The signature change is in the linked issue so should be straightforward. I can deal with that if you would prefer not to.

[lavafroth]

I have committed the suggested changes.
Edit: I haven't made changes to CHANGELOG.rst, doc_src/cmds/string-repeat.rst or doc_src/cmds/string.rst.

[henrikhorluck]

Merged, thanks!