prompt: fix missing file errors in zsh

prompt: fix missing file errors in zsh

zsh seems to have a bug while redirecting the stderr of the 'read'
command:

    % read foo 2>/dev/null <foo
    zsh: no such file or directory: foo

Which causes errors to be displayed when certain files are missing.
Let's add a convenience function to manually check if the file is
readable before calling "read".

Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>