git.CatPipe: more resilience against weird errors.

git.CatPipe: more resilience against weird errors.

Notably, MemoryErrors thrown because the file we're trying to load into
memory is too big to load all at once.  Now the MemoryError gets thrown, but
the main program is potentially able to recover from it because CatPipe at
least doesn't get into an inconsistent state.

Also we can recover nicely if some lamer kills our git-cat-file subprocess.

The AutoFlushIter we were using for this purpose turns out to not have been
good enough, and it's never been used anywhere but in CatPipe, so I've
revised it further and renamed it to git.AbortableIter.