"reset --merge": fix unmerged case

"reset --merge": fix unmerged case

Commit 9e8ecea (Add 'merge' mode to 'git reset', 2008-12-01) disallowed
"git reset --merge" when there was unmerged entries.  But it wished if
unmerged entries were reset as if --hard (instead of --merge) has been
used.  This makes sense because all "mergy" operations makes sure that
any path involved in the merge does not have local modifications before
starting, so resetting such a path away won't lose any information.

The previous commit changed the behavior of --merge to accept resetting
unmerged entries if they are reset to a different state than HEAD, but it
did not reset the changes in the work tree, leaving the conflict markers
in the resulting file in the work tree.

Fix it by doing three things:

 - Update the documentation to match the wish of original "reset --merge"
   better, namely, "An unmerged entry is a sign that the path didn't have
   any local modification and can be safely resetted to whatever the new
   HEAD records";

 - Update read_index_unmerged(), which reads the index file into the cache
   while dropping any higher-stage entries down to stage #0, not to copy
   the object name from the higher stage entry.  The code used to take the
   object name from the a stage entry ("base" if you happened to have
   stage #1, or "ours" if both sides added, etc.), which essentially meant
   that you are getting random results depending on what the merge did.

   The _only_ reason we want to keep a previously unmerged entry in the
   index at stage #0 is so that we don't forget the fact that we have
   corresponding file in the work tree in order to be able to remove it
   when the tree we are resetting to does not have the path.  In order to
   differentiate such an entry from ordinary cache entry, the cache entry
   added by read_index_unmerged() is marked as CE_CONFLICTED.

 - Update merged_entry() and deleted_entry() so that they pay attention to
   cache entries marked as CE_CONFLICTED.  They are previously unmerged
   entries, and the files in the work tree that correspond to them are
   resetted away by oneway_merge() to the version from the tree we are
   resetting to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>