Add completions/src.fish

[adiabatic]

Description

I had a look through ~/.local/share/fish/generated_completions/src.fish and it was clear to me that fish_update_completions was very confused by the src manpage. While I didn't add code to handle the -<n> syntax or provide lists of tags and branches for tag/branch deletion, I can confidently say that I'm less confused by the manpage than fish_update_completions was.

Hope this is useful to other people.

TODOs:

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

[faho]

Thanks, merged!