update: better handle merge conflicts.

When there are merge conflicts we fail pretty hard. This is still
possible after this commit but at least we've given Git enough pointers
to make it less likely.

Closes Homebrew/homebrew#49299.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/update.sh

@@ -203,6 +203,7 @@ pull() {
       echo "Stashing uncommitted changes to $DIR."
       git status --short --untracked-files=all
     fi
+    git merge --abort &>/dev/null
     git -c "user.email=brew-update@localhost" \
         -c "user.name=brew update" \
         stash save --include-untracked "${QUIET_ARGS[@]}"
@@ -232,7 +233,10 @@ pull() {
   then
     git rebase "${QUIET_ARGS[@]}" "origin/$UPSTREAM_BRANCH"
   else
-    git merge --no-edit --ff "${QUIET_ARGS[@]}" "origin/$UPSTREAM_BRANCH"
+    git merge --no-edit --ff "${QUIET_ARGS[@]}" "origin/$UPSTREAM_BRANCH" \
+      --strategy=recursive \
+      --strategy-option=ours \
+      --strategy-option=ignore-all-space
   fi
 
   export HOMEBREW_UPDATE_AFTER"$TAP_VAR"="$(read_current_revision)"