diff --no-index: support reading from named pipes

diff --no-index: support reading from named pipes

In some shells, such as bash and zsh, it's possible to use a command
substitution to provide the output of a command as a file argument to
another process, like so:

  diff -u <(printf "a\nb\n") <(printf "a\nc\n")

However, this syntax does not produce useful results with "git diff
--no-index". On macOS, the arguments to the command are named pipes
under /dev/fd, and git diff doesn't know how to handle a named pipe. On
Linux, the arguments are symlinks to pipes, so git diff "helpfully"
diffs these symlinks, comparing their targets like "pipe:[1234]" and
"pipe:[5678]".

To address this "diff --no-index" is changed so that if a path given on
the commandline is a named pipe or a symbolic link that resolves to a
named pipe then we read the data to diff from that pipe. This is
implemented by generalizing the code that already exists to handle
reading from stdin when the user passes the path "-".

If the user tries to compare a named pipe to a directory then we die as
we do when trying to compare stdin to a directory.

As process substitution is not support by POSIX this change is tested by
using a pipe and a symbolic link to a pipe.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>