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.
