Rename gh alias (git help) to ghh

This conflicts with several tools, including NodeGH [1] and
scm_breeze [2].

[1] #5313
[2] #5321

Related: #5130

plugins/git/git.plugin.zsh

@@ -151,7 +151,7 @@ compdef _git ggpush=git-checkout
 alias ggsup='git branch --set-upstream-to=origin/$(git_current_branch)'
 alias gpsup='git push --set-upstream origin $(git_current_branch)'
 
-alias gh='git help'
+alias ghh='git help'
 
 alias gignore='git update-index --assume-unchanged'
 alias gignored='git ls-files -v | grep "^[[:lower:]]"'