Clear the localizability warning in the autocrlf pop up - #594

The core.autocrlf pop ups carry one row that means "no value", and the
controller recognised it by comparing against its (not set) title.
That made a piece of user-facing text load bearing, which is what the
NonLocalizedStringChecker analyzer warning was pointing at.

• Mark that row in the nib with a tag and find it by that tag, so the
  words on it no longer decide what gets saved
• Read the setting from the selected title only once the tag has ruled
  out the unset row, which is safe because every other row is titled
  with a value git accepts
• Cover both pop ups with a test, so the nib and the controller cannot
  drift apart quietly

Wrapping the literal in NSLocalizedString would have cleared the
warning and left a trap behind. GitX ships only en.lproj, so the macro
returns its key unchanged and everything keeps working; the day a second
locale arrives, the controller's string and the nib's title diverge,
itemWithTitle: stops matching, and GitX writes the translated words
into core.autocrlf. Keying on a tag instead frees the label to be
translated whenever that day comes.

The one thing the change moves into the nib is the tag itself, which is
why the tests assert on the nib rather than trusting it.

Test plan

• Two new tests fail before the nib change and pass after it, covering
  the repository and the global pop up
• GitXTests is green at 47 of 47
• xcodebuild analyze no longer reports the warning at
  PBGitPrefsWindowController.m:169, with the file confirmed
  re-analyzed in that run