Reset Command Exec Time

Reset the `cmd_timestamp` immediately after printing to avoid printing erroneous times on subsequent empty prompts.

themes/refined.zsh-theme

@@ -72,6 +72,7 @@ preexec() {
 precmd() {
     vcs_info # Get version control info before we start outputting stuff
     print -P "\n$(repo_information) %F{yellow}$(cmd_exec_time)%f"
+    unset cmd_timestamp #Reset cmd exec time.
 }
 
 # Define prompts