t0081 (line-buffer): add buffering tests

POSIX makes the behavior of read(2) from a pipe fairly clear: a read
from an empty pipe will block until there is data available and any
other read will not block, prefering to return a partial result.
Likewise, fread(3) and fgets(3) are clearly specified to act as
though implemented by calling fgetc(3) in a simple loop.  But the
buffering behavior of fgetc is less clear.

Luckily, no sane platform is going to implement fgetc by calling the
equivalent of read(2) more than once.  fgetc has to be able to
return without filling its buffer to preserve errno when errors are
encountered anyway.  So let's assume the simpler behavior (trust) but
add some tests to catch insane platforms that violate that when they
come (verify).

First check that fread can handle a 0-length read from an empty fifo.
Because open(O_RDONLY) blocks until the writing end is open, open the
writing end of the fifo in advance in a subshell.

Next try short inputs from a pipe that is not filled all the way.

Lastly (two tests) try very large inputs from a pipe that will not fit
in the relevant buffers.  The first of these tests reads a little
more than 8192 bytes, which is BUFSIZ (the size of stdio's buffers)
on this Linux machine.  The second reads a little over 64 KiB (the
pipe capacity on Linux) and is not run unless requested by setting
the GIT_REMOTE_SVN_TEST_BIG_FILES environment variable.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

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