Fix a bug in the default git_prompt_info ()

plugins/git/git-prompt.plugin.zsh

@@ -26,6 +26,11 @@
 #   # with your format:
 #   git_prompt_info ()
 #   {
+#     if [ -z "$(git_prompt__git_dir)" ]; then
+#       GIT_PROMPT_INFO=''
+#       return
+#     fi
+#
 #     git_prompt__branch
 #     local branch_=$GIT_PROMPT_BRANCH
 #   
@@ -63,13 +68,17 @@
 # (See the ashleydev theme for more complex usage).
 git_prompt_info ()
 {
+  if [ -z "$(git_prompt__git_dir)" ]; then
+    GIT_PROMPT_INFO=''
+    return
+  fi
+
   git_prompt__branch
   local branch=$GIT_PROMPT_BRANCH
 
   git_prompt__dirty_state
   local dirty=$GIT_PROMPT_DIRTY_STATE_ANY_DIRTY
 
-  if [[ -n "$branch" ]]; then
   local prompt=$branch
 
   if [[ "$dirty" = 'yes' ]]; then
@@ -79,7 +88,6 @@ git_prompt_info ()
   fi
 
   GIT_PROMPT_INFO="$ZSH_THEME_GIT_PROMPT_PREFIX$prompt$ZSH_THEME_GIT_PROMPT_SUFFIX"
-  fi
 }
 
 #------------------ git information utils ------------------