usage: do not insist that standard input must come from a file

usage: do not insist that standard input must come from a file

The synopsys text and the usage string of subcommands that read list
of things from the standard input are often shown like this:

	git gostak [--distim] < <list-of-doshes>

This is problematic in a number of ways:

 * The way to use these commands is more often to feed them the
   output from another command, not feed them from a file.

 * Manual pages outside Git, commands that operate on the data read
   from the standard input, e.g "sort", "grep", "sed", etc., are not
   described with such a "< redirection-from-file" in their synopsys
   text.  Our doing so introduces inconsistency.

 * We do not insist on where the output should go, by saying

	git gostak [--distim] < <list-of-doshes> > <output>

 * As it is our convention to enclose placeholders inside <braket>,
   the redirection operator followed by a placeholder filename
   becomes very hard to read, both in the documentation and in the
   help text.

Let's clean them all up, after making sure that the documentation
clearly describes the modes that take information from the standard
input and what kind of things are expected on the input.

[jc: stole example for fmt-merge-msg from Jonathan]

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>