fish: add a function for cloning colocated repos

I've hit too many issues in pure JJ where I end up getting wedged,
*plus* so many tools I use have great functions for working with active
Git repos, but not with JJ. Simplest solution: make cloning a
colocated repo the easy path.

fish/functions/gjc.fish

···
+function gjc --description 'git clone a repo and immediately jj initialize it' --argument-names source dest branch
+    if test -z "$dest"
+        set dest (string split / "$source")[-1]
+    end
+    git clone $source $dest
+    cd $dest
+    jj git init --colocate
+    if test -z "$branch"
+        set branch (git rev-parse --abbrev-ref HEAD)
+    end
+    jj bookmark track {$branch}@origin
+end