Fix git completions when aliased command is not found.

The "__fish_git_using_command" function in "share/completions/git.fish" fetches the complete name of an aliased git command through "git config --get". If the alias is not found, git config prints an error string on stderr instead of stdout. Since __fish_git_using_command is called several times, if the alias is not found the terminal is flooded with those error messages. I redirected stderr to /dev/null to fix this.

In order to reproduce the bug, type something like:
git -n\<spacebar\>\<tab\>

error: invalid key: alias.-n
error: invalid key: alias.-n
(...)