add diff when updating cache manually

[amtoine]

let's say i update repos of my store manually:

• i git init a repo called manual/foo
• i git init a repo called manual/bar
• i rm a repo called auto/baz

when i run gm update-cache to remove auto/baz from it and add manual/foo and manual/bar to it, i think it would be cool to have an output like the following:

> gm update-cache
updating cache... done
#┬───name───┬status─
0│manual/foo│added
1│manual/bar│added
2│auto/baz  │removed
─┴──────────┴───────

this implies that the signature of gm update-cache would change a bit from

"gm update-cache" []: nothing -> nothing

to

"gm update-cache" []: nothing -> table<name: string, status: string>