Fix the first lines of a file hiding behind its tab bar - #605

Fixes #598.

The Source, Blame and History bar shares its pane with the file viewer:
the bar sits in the top 25 points, the viewer fills the rest. Ending a
live window resize threw that away:

- (void)windowDidEndLiveResizeWithNotification:(NSNotification *)theNotification
{
    self.view.autoresizingMask = ...;
    self.view.frame = self.view.superview.bounds;   // the whole pane
}

The viewer then covered the bar's 25 points too, and since the bar is the
later sibling it is drawn on top, hiding the first line and a half of
every file from then on. That is why it looks like a per-repository
problem: it is per-window, and it starts the first time that window is
resized.

PBWebController now asks itself which frame its web view is to occupy,
which for every other web view in GitX is still the whole superview, and
GLFileView answers with the room below the bar.

Test plan

• New GLFileViewLayoutTests: 3 tests, all failing on master, covering
  the viewer's frame at two pane heights and with a bar laid out taller
  than the nib reserved for it
• make unit-test: 52 tests, 0 failures
• Traced the real thing: with a file open in the tree view, the viewer
  sits at y=546, below the bar at 521..546. Dragging the window edge on
  master moves it to y=521, under the bar; with this change it stays at
  546 across repeated resizes
• Checked the commit view's diff, which shares the base class, still
  fills its own pane after a resize