zsh: Move 'change directory into repo root' logic

Add an alias that switches directories into the repository root, instead
of having the normal `cd` command behave like that.

Sadly this is not possible (AFAIK) with a git alias as that will always
spawn a subshell.

.config/zsh/zshrc.d/30-alias.zsh

@@ -60,6 +60,7 @@ fi
 	alias ga='git add'
 	alias gap='git add -p'
 	alias gc='git commit'
+	alias gcd='cd "$(git rev-parse --show-toplevel)"'
 	alias gch='git checkout'
 	alias gd='git diff'
 	alias gds='git diff --staged'