Fix the gitx stdin check leaking the stream it opens - #593

The command line tool decides whether to read a piped diff by opening a stream
on stdin and throwing the result away, so every run that is not attached to a terminal
leaks a FILE for the life of the process.
This is the unix.Stream analyzer finding in gitx.m.

• Test that stdin is open and readable through its descriptor flags
  instead, which answers the same question without allocating anything
• Keep the guard in place, since handleSTDINDiff reads stdin through
  NSFileHandle and raises when the descriptor is closed

The stream cannot simply be closed: fclose would take stdin with it,
and handleSTDINDiff needs that same descriptor. The guard cannot be
dropped either, because isatty returns 0 for a closed stdin, so it is
the guard that keeps readDataToEndOfFile off a bad descriptor.
The new check matches fdopen(fd, "r") exactly, which fails both on an invalid
descriptor and on a write-only one.

Test plan

• xcodebuild analyze no longer reports the unix.Stream warning at gitx.m:370,
  with gitx.m confirmed re-analyzed in that run
• Run from a non-repository directory, gitx gets past the guard and
  prints its usual message under an empty pipe, a closed stdin,
  a /dev/null stdin and a write-only descriptor
• A pipe held open for three seconds still blocks in handleSTDINDiff
  for the full three, while a closed stdin returns at once, so the
  readable path is still entered rather than skipped