rev-parse: better error message for ambiguous arguments

Currently, if git-rev-parse encounters an argument that is neither a
recognizable revision name nor the name of an existing file or
directory, and it hasn't encountered a "--" argument, it prints an
error message saying "No such file or directory".  This can be
confusing for users, including users of programs such as gitk that
use git-rev-parse, who may then think that they can't ask about the
history of files that no longer exist.

This makes it print a better error message, one that points out the
ambiguity and tells the user what to do to fix it.

Signed-off-by: Paul Mackerras <paulus@samba.org>