cmd/update: stop Git's fsmonitor when needed

Git's fsmonitor daemon will prevent our update lock file from being
released if it is running before a `brew update`.

Let's fix that by stopping it whenever necessary so our update lock is
released upon completion.

Fixes #13521.

Library/Homebrew/cmd/update.sh

@@ -556,6 +556,12 @@ EOS
     [[ -d "${DIR}/.git" ]] || continue
     cd "${DIR}" || continue
 
+    # Git's fsmonitor daemon will not release our lock unless we stop it.
+    if git fsmonitor--daemon status &>/dev/null
+    then
+      git fsmonitor--daemon stop 2>/dev/null
+    fi
+
     if ! git config --local --get remote.origin.url &>/dev/null
     then
       opoo "No remote 'origin' in ${DIR}, skipping update!"