Prevent user-defined aliases/commands checking (#785)

#748 
There's no stdout/stderr on this condition.

.config/aliasrc

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # Use neovim for vim if present.
-command -v nvim >/dev/null && alias vim="nvim" vimdiff="nvim -d"
+[ -x "$(command -v nvim)" ] && alias vim="nvim" vimdiff="nvim -d"
 
 # Verbosity and settings that you pretty much just always are going to want.
 alias \