Fix git_commits_{ahead,before} when no upstream branch is defined (#6658)

If @{u} is not defined, git rev-list will give an error; redirect
to stderr the error and deal with this case in what follows.

lib/git.zsh

@@ -77,8 +77,8 @@ function git_current_branch() {
 # Gets the number of commits ahead from remote
 function git_commits_ahead() {
   if command git rev-parse --git-dir &>/dev/null; then
-    local commits="$(git rev-list --count @{upstream}..HEAD)"
-    if [[ "$commits" != 0 ]]; then
+    local commits="$(git rev-list --count @{upstream}..HEAD 2>/dev/null)"
+    if [[ -n "$commits" && "$commits" != 0 ]]; then
       echo "$ZSH_THEME_GIT_COMMITS_AHEAD_PREFIX$commits$ZSH_THEME_GIT_COMMITS_AHEAD_SUFFIX"
     fi
   fi
@@ -87,8 +87,8 @@ function git_commits_ahead() {
 # Gets the number of commits behind remote
 function git_commits_behind() {
   if command git rev-parse --git-dir &>/dev/null; then
-    local commits="$(git rev-list --count HEAD..@{upstream})"
-    if [[ "$commits" != 0 ]]; then
+    local commits="$(git rev-list --count HEAD..@{upstream} 2>/dev/null)"
+    if [[ -n "$commits" && "$commits" != 0 ]]; then
       echo "$ZSH_THEME_GIT_COMMITS_BEHIND_PREFIX$commits$ZSH_THEME_GIT_COMMITS_BEHIND_SUFFIX"
     fi
   fi