feat(lib): show upstream branch in `git_prompt_info` (#9188)

Show the remote branch the local branch is tracking if `ZSH_THEME_GIT_SHOW_UPSTREAM`
is set, like so: `ZSH_THEME_GIT_SHOW_UPSTREAM=1`.
Co-authored-by: Marc Cornellà <marc.cornella@live.com>

lib/git.zsh

@@ -9,14 +9,27 @@ function __git_prompt_git() {
   GIT_OPTIONAL_LOCKS=0 command git "$@"
 }
 
-# Outputs current branch info in prompt format
 function git_prompt_info() {
+  # If we are on a folder not tracked by git, get out.
+  # Otherwise, check for hide-info at global and local repository level
+  if ! __git_prompt_git rev-parse --git-dir &> /dev/null \
+     || [[ "$(__git_prompt_git config --get oh-my-zsh.hide-info 2>/dev/null)" == 1 ]]; then
+    return 0
+  fi
+
   local ref
-  if [[ "$(__git_prompt_git config --get oh-my-zsh.hide-status 2>/dev/null)" != "1" ]]; then
-    ref=$(__git_prompt_git symbolic-ref HEAD 2> /dev/null) || \
-    ref=$(__git_prompt_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"
+  ref=$(__git_prompt_git symbolic-ref --short HEAD 2> /dev/null) \
+  || ref=$(__git_prompt_git rev-parse --short HEAD 2> /dev/null) \
+  || return 0
+ 
+  # Use global ZSH_THEME_GIT_SHOW_UPSTREAM=1 for including upstream remote info
+  local upstream
+  if (( ${+ZSH_THEME_GIT_SHOW_UPSTREAM} )); then
+    upstream=$(__git_prompt_git rev-parse --abbrev-ref --symbolic-full-name "@{upstream}" 2>/dev/null) \
+    && upstream=" -> ${upstream}"
   fi
+
+  echo "${ZSH_THEME_GIT_PROMPT_PREFIX}${ref}${upstream}$(parse_git_dirty)${ZSH_THEME_GIT_PROMPT_SUFFIX}"
 }
 
 # Checks if working tree is dirty