Fix a branch checkout leaving its tip off screen - #612

Fixes #606.

Changing branch does move the history list to the new branch's tip, but
the rebuild that follows the checkout puts the scroll back at the top and
drops the selection along with it. Restoring that selection put it back
where it stood without bringing it into view, so on any repository whose
branch tips are more than a screen apart the list looked untouched.

Traced on a 60-commit repository, switching to a branch whose tip is row
56 of the list:

scroll oldIndex=55 newIndex=55   visible={{0, -28}, ...}  ->  {{0, 791}, ...}   moved to the tip
restore  currentlySelected=1     visible={{0, 791}, ...}
restore  currentlySelected=0     visible={{0, -28}, ...}   rebuild: selection and scroll both lost
restore  PUTTING BACK selection  visible={{0, -28}, ...}   selection back, scroll left at the top

This comes from #579, not from #582: before it, the
rebuild's dropped selection was re-made by -selectCommit:, which
scrolled as it went. -gitDirForURL: is only read when a document is
opened, and the string it builds is the same one as before - the old code
ran stringWithUTF8String: over the buffer, which stops at the same NUL
that size counts up to.

Test plan

• PBGitHistorySelectionTests gains 2 tests, of which
  testARestoredSelectionIsBroughtBackIntoView fails on master
• make unit-test: 51 tests, 0 failures
• On that 60-commit repository, double-clicking the far branch used to
  leave row 1 at the top of the view; it now scrolls so the selected row
  56 sits inside the viewport