[git] Use fetch --all in gfu alias

instead of `remote update`. Both behave the same way, fetching from all
configured remotes. The `fetch --all` is shorter, and probably more
familiar. See https://stackoverflow.com/a/17512004/2654518

Also change `;` to `&&`, so merge is only called after fetch succeeded.

Closes #196

modules/git/init.zsh

@@ -71,7 +71,7 @@ alias gf='git fetch'
 alias gfc='git clone'
 alias gfm='git pull'
 alias gfr='git pull --rebase'
-alias gfu='git remote update --prune; git merge --ff-only @\{u\}'
+alias gfu='git fetch --all --prune && git merge --ff-only @\{u\}'
 
 # Grep (g)
 alias gg='git grep'