Clean up different special *HEAD handling

Clean up different special *HEAD handling

We codify the following different heads (in addition to the main "HEAD",
which points to the current branch, of course):

 - FETCH_HEAD

   Populated by "git fetch"

 - ORIG_HEAD

   The old HEAD before a "git pull/resolve" (successful or not)

 - LAST_MERGE

   The HEAD we're currently merging in "git pull/resolve"

 - MERGE_HEAD

   The previous head of a unresolved "git pull", which gets committed by
   a "git commit" after manually resolving the result

We used to have "MERGE_HEAD" be populated directly by the fetch, and we
removed ORIG_HEAD and LAST_MERGE too aggressively.