agnoster: fix git working tree check (#9072)

`git rev-parse --is-inside-work-tree` does not have a different exit
code when not within a work tree; the difference is in the output,
either "true" when in a work tree, or "false" when not.

The if was modified similarly to the check for the hide-status config,
which also parses command output rather than exit code.
Co-authored-by: Kyle Laker <klaker@easydynamics.com>

themes/agnoster.zsh-theme

@@ -106,7 +106,7 @@ prompt_git() {
   }
   local ref dirty mode repo_path
 
-  if $(git rev-parse --is-inside-work-tree >/dev/null 2>&1); then
+   if [[ "$(git rev-parse --is-inside-work-tree 2>/dev/null)" = "true" ]]; then
     repo_path=$(git rev-parse --git-dir 2>/dev/null)
     dirty=$(parse_git_dirty)
     ref=$(git symbolic-ref HEAD 2> /dev/null) || ref="➦ $(git rev-parse --short HEAD 2> /dev/null)"