Fix phantom "detached HEAD" entry stuck in the sidebar - #569

Rebasing a series of local branches from a terminal while GitX watched the
repository left a "detached HEAD" row permanently at the top of the BRANCHES
list, long after every rebase had finished and HEAD was attached again.
Selecting other branches did not clear it, because by then the row was no
longer backed by the model at all.

The full chain, confirmed by instrumenting reloadRefs and the sidebar:

1. reloadRefs injects a synthetic "HEAD" ref while isHEADDetached is true,
   which git rebase makes true for its whole run. An FSEvent refresh landing
   mid-rebase therefore adds a legitimate, transient "detached HEAD" row.
   Its title starts with a curly quote, so it sorts first in BRANCHES.

2. The sidebar's currentBranch and refs observers captured selectedRow as an
   index, called reloadData, then re-selected that same index. reloadData
   drops the selection entirely (observed: selectedRow becomes -1), so the
   forced re-select always fires a selection change - and because the new row
   was inserted at the top of BRANCHES, the stale index now denoted a
   different item. With the first branch row selected, that item was the
   phantom itself, so outlineViewSelectionDidChange: assigned
   currentBranch = "HEAD" with no user interaction.

3. When the rebase finished, reloadRefs correctly pruned "HEAD" and the
   branches KVO removal deleted the row, but the prune loop only guarded
   headRef, never currentBranch. Nothing else reconciles currentBranch after
   an external HEAD change: the FSEvent refresh path calls reloadRefs and
   only reloadRefs, while readCurrentBranch is invoked solely by GitX's own
   HEAD-moving operations. currentBranch was left dangling at a spec absent
   from repository.branches.

4. selectCurrentBranch fed that dangling spec into addRevSpec:, rebuilding
   the row as an orphan. The sidebar only deletes rows in response to a
   branches KVO removal, and the model had already removed this one, so no
   removal could ever fire for it again.

Changes:

- Re-select by item instead of by row index in both sidebar observers, via a
  new reloadPreservingSelection helper. This keeps 6e92c1a's intent of
  recomputing the checked-out badge on every refresh, without reassigning the
  user's selected branch behind their back.
- Reset a pruned currentBranch through readCurrentBranch at the end of
  reloadRefs, so an externally moved or deleted HEAD heals itself. Guarded on
  the branch actually having been pruned, since setCurrentBranch: reaches
  reloadRefs again through updateHistory.
- Look the row up with itemForRev: in selectCurrentBranch, so a sidebar row
  can never outlive its model entry. Row creation stays in the branches KVO
  insertion path.
- Call readCurrentBranch after reloadRefs in pullBranch:fromRemote:rebase:,
  matching checkout, merge, cherry-pick, reset and rebase. GitX's own Pull
  and Rebase runs on a background queue, so it has the same mid-rebase window.

Verified against a scratch repo: an external detach/re-attach, deleting the
selected branch externally, a five branch external pull --rebase loop, and a
conflicted rebase followed by --abort. The row still appears while HEAD is
genuinely detached, always disappears afterwards, currentBranch never dangles,
and the sidebar selection stays put throughout.