Improve speed of HOMEBREW_AUTO_UPDATE_SECS

This variable allows a user to configure how often `brew update` runs.
With the previous implementation, however, it was still pretty slow as
it would run some of `brew update` even within the timeout. This feels
related to complaints about Homebrew "feeling slow" since we enabled
this. Address this by:

- making `HOMEBREW_AUTO_UPDATE_SECS` behave more like
  `HOMEBREW_NO_AUTO_UPDATE` and entirely skip the invocation of
  `brew update` if the relevant tap (homebrew-core or homebrew-cask
  depending on command invoked) has been checked in the last
  `HOMEBREW_AUTO_UPDATE_SECS`
- Only require/check the update of a single tap rather than every tap
- Increase the default value of `HOMEBREW_AUTO_UPDATE_SECS` from 1m
  to 5m to cut people a bit more slack. We're not updating things
  often enough for 1m not to feel a bit overkill and 5m feels
  appropriate for a Homebrew "session" to only require a single update.

Library/Homebrew/cmd/update.sh

@@ -337,7 +337,7 @@ EOS
 
   if [[ -z "$HOMEBREW_AUTO_UPDATE_SECS" ]]
   then
-    HOMEBREW_AUTO_UPDATE_SECS="60"
+    HOMEBREW_AUTO_UPDATE_SECS="300"
   fi
 
   # check permissions
@@ -451,12 +451,6 @@ EOS
     # origin branch name is, and use that. If not set, fall back to "master".
     # the refspec ensures that the default upstream branch gets updated
     (
-      if [[ -n "$HOMEBREW_UPDATE_PREINSTALL" ]]
-      then
-        # Skip taps checked/fetched recently
-        [[ -n "$(find "$DIR/.git/FETCH_HEAD" -type f -mtime -"${HOMEBREW_AUTO_UPDATE_SECS}"s 2>/dev/null)" ]] && exit
-      fi
-
       UPSTREAM_REPOSITORY_URL="$(git config remote.origin.url)"
       if [[ "$UPSTREAM_REPOSITORY_URL" = "https://github.com/"* ]]
       then