Allow modules that are not repos

so going into their ${ZIM_HOME}/modules/foo directory would be like
still being inside the Zim repo, in ${ZIM_HOME}. Don't try to update
them in this case.

This was supposed to be working before, but my ${ZIM_HOME} was not a
repo when I was still developing this locally.

tools/update.zsh

@@ -9,8 +9,8 @@ if ! cd ${DIR} 2>/dev/null; then
   print -P "%F{red}✗ ${MODULE}: Not installed%f"
   return 1
 fi
-if ! command git rev-parse --is-inside-work-tree &>/dev/null; then
-  # Not a git repository. Will not try to update.
+if [[ ${PWD} != $(command git rev-parse --show-toplevel 2>/dev/null) ]]; then
+  # Not in repo root. Will not try to update.
   return 0
 fi
 if [[ ${URL} != $(command git config --get remote.origin.url) ]]; then