bug#18449: "cat x >> x" error even when x is empty

[Paul Eggert]

Eric Blake wrote:
> Testing for an empty file is enough of an additional special case over
> the existing check for same files that I don't think it is worth it.

It's a really cheap check, as a system call is needed only when the 
input and output files are the same, and even then it's only an lseek 
with SEEK_CUR so it's all in-memory.  Plus, the POSIX 'cat' spec gives 
an example of using 'cat' to copy an empty regular file to itself, and 
(not unreasonably) says that should work.

As far as POSIX conformance goes, I don't think we need to worry about 
POSIXLY_CORRECT here.  We'll get POSIX fixed instead.  Eric has already 
started the ball rolling on that (thanks).  Though the POSIX fix should 
involve updating its example -- and quite possibly POSIX should continue 
to allow catting an empty file to itself.

I installed the attached patch and am marking this as done.  The 'cat' 
source code is simpler now, so that's a win anyway....

0001-cat-allow-copying-empty-files-to-themselves.patch
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