git:completion: Fix command not found error

_git has to be loaded before other functions can use its completion
functions.

Hint to this found at: https://unix.stackexchange.com/a/269818

.config/zsh/zshrc.d/20-completion.zsh

@@ -38,6 +38,10 @@ compdef nvim-man=man
 # Copy git completion for arguments
 compdef _git-log glog
 compdef _git-checkout git-checkout-worktree
+# Run git's completion once to avoid a `command not found` error when using the
+# completion functions on other programs without having completed something for
+# git before in the same session.
+_git &>/dev/null || true
 
 # Do not sort `git checkout`s completion
 zstyle ":completion:*:git-checkout:*" sort false