git: Try a bit harder not to lose errno in stdio

git: Try a bit harder not to lose errno in stdio

This switches the checks around upon the exit codepath of the
git wrapper, so that we may recover at least non-transient errors.

It's still not perfect. As I've been harping on, stdio simply isn't very
good for error reporting. For example, if an IO error happened, you'd want
to see EIO, wouldn't you? And yes, that's what the kernel would return.
However, with buffered stdio (and flushing outside of our control), what
would likely happen is that some intermediate error return _does_ return
EIO, but then the kernel might decide to re-mount the filesystem read-only
due to the error, and the actual *report* for us might be

	"write failure on standard output: read-only filesystem"

which lost the EIO.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>