To avoid recording unrelated changes in the merge commit,
'git pull' and 'git merge' will also abort if there are any changes
-registered in the index relative to the `HEAD` commit. (One
-exception is when the changed index entries are in the state that
-would result from the merge already.)
+registered in the index relative to the `HEAD` commit. (Special
+narrow exceptions to this rule may exist depending on which merge
+strategy is in use, but generally, the index must match HEAD.)
If all named commits are already ancestors of `HEAD`, 'git merge'
will exit early with the message "Already up to date."