git_prompt_info: ignore git-config errors

Merged from #2906

The function `git_prompt_info` calls `git config` for its stdout output,
but doesn't handle the stderr output.  This can lead to problems,
e.g. if the git config file is unreadable for some reason (permissions
etc).

This fixes the issue by simply ignoring the stderr output.

lib/git.zsh

 # get the name of the branch we are on
 function git_prompt_info() {
-  if [[ "$(command git config --get oh-my-zsh.hide-status)" != "1" ]]; then
+  if [[ "$(command git config --get oh-my-zsh.hide-status 2>/dev/null)" != "1" ]]; then
     ref=$(command git symbolic-ref HEAD 2> /dev/null) || \
     ref=$(command git rev-parse --short HEAD 2> /dev/null) || return 0
     echo "$ZSH_THEME_GIT_PROMPT_PREFIX${ref#refs/heads/}$(parse_git_dirty)$ZSH_THEME_GIT_PROMPT_SUFFIX"