Add Fossil support to fish_vcs_prompt

[nat-418]

Currently, fossil-scm is supported for fish completions, but not in, e.g., the informative-vcs prompt.

fish, version 3.5.1
Linux laptop 5.15.86 #1-NixOS SMP Sat Dec 31 12:14:48 UTC 2022 x86_64 GNU/Linux
xterm-256color
I tried running fish without third-party customizations and it did not affect behavior.

To reproduce, type fish_vcs_prompt in a fossil repository checkout directory.

[faho]

This is a textbook example of "patches welcome". We're gonna need someone who knows fossil to write something.

[nat-418]

I am working on it. For now I am still trying to figure out how fish_vcs_prompt works. Adding support for Fossil should be pretty easy given that we have fossil info and fossil status that give us all the information we need in a simple key-value format.

[faho]

fish_vcs_prompt is just a cover function that goes through all the known vcsen.

It simply calls fish_git_prompt for git and fish_hg_prompt for mercurial.

What you want is this:

A file called fish_fossil_prompt.fish, with a function called fish_fossil_prompt (i.e. the same name as the file, no .fish ending) that functions as the entry point. You can add more functions inside the file, but that is the one that ends up being called.

Then, you want the function to at the very least have this:

function fish_fossil_prompt
    if not type -q fossil
        # no fossil installed, we gracefully fail
        return 127
    end
    
    # do whatever to get the fossil info
    set -l info (fossil info)
    # *jazz hands* MAGIC */jazz hands*

    echo -- $fossil_branch $fossil_repostate # I don't know fossil, put whatever useful info here
end

You basically want to look at the hg prompt, not the git one because that's overly complicated for legacy reasons (especially the fifty million configuration variables it exposes).

If this prompt is fast enough, we can add it to fish_vcs_prompt to enable it by default. It especially needs to be very quick (on the order of single-digit milliseconds) outside of a fossil repo, even if fossil is installed.

If fossil itself is slow at figuring out if it is a fossil repo or not, you can do what the hg prompt does (mercurial is quite slow in starting up, so we don't want to pay that cost for everyone on every prompt), and figure it out yourself (mercurial looks for a ".hg" directory with a "dirstate" file IIRC).

If it still isn't fast enough, we can accept it like the svn prompt - as off by default.