Fix the staging screenshot flaking on the commit message caret - #716

Take the keyboard focus out of the commit message field before the
staging view is captured, so the comparison stops reporting a caret
that no pull request drew.

PBGitCommitController's first responder is the commit message field, so
opening the staging view puts the keyboard focus there and its insertion
point blinks. Whether the caret lands in the capture then comes down to
when the screenshot is taken, and the comparison posts a difference
nobody made - most recently on #713.

• Click the diff pane, which reads "No file selected" at that point and
  so takes the focus without selecting or changing anything.
• Assert the diff pane is present first, so a view that never loaded
  fails with its own message rather than as a click into nothing.

Tab is not a way out here: the text view's doCommandBySelector: hands
the focus to a table, and focusTable: no-ops on an empty one, which is
what the fixture repo shows.

The second commit covers a different way a capture can differ from one
taken a moment later: a window whose app is not frontmost has its title
bar drawn dimmed. The tests assume the launch left the app in front
rather than checking, so the capture now waits for that, and asks for
activation only when the app is not already there - an activation
request would dismiss the open context menu the third test captures.

Test plan

• Ran testStagingTabScreenshot locally six times before and after: the
  caret is in 5 of 5 captures before the change and 0 of 7 after
  (6 single runs plus a full-suite run), measured by scanning the commit
  message area for the caret's colour.
• The whole GitXUITests suite passes, all four tests.
• This pull request's own comparison should show the caret leaving the
  staging view once; comparisons after it should be quiet.
• For the activation commit, reproduced the symptom by handing the
  activation to Finder just before the history capture: the title bar
  comes back dimmed and the traffic lights grey, and with the wait in
  place it matches an undisturbed run (title glyph brightness 105.9
  against 105.8).
• Suite green again with both commits, and all three captures render
  the title identically.