guix hash: Interpret '-' as standard input.

* guix/scripts/hash.scm (guix-hash)[file-hash]: Special-case "-".
* tests/guix-hash.sh: Add test.
* doc/guix.texi (Invoking guix hash): Document it.

doc/guix.texi

@@ -4791,7 +4791,9 @@ The general syntax is:
 guix hash @var{option} @var{file}
 @end example
 
-@command{guix hash} has the following options:
+When @var{file} is @code{-} (a hyphen), @command{guix hash} computes the
+hash of data read from standard input.  @command{guix hash} has the
+following options:
 
 @table @code
 

guix/scripts/hash.scm

@@ -139,7 +139,9 @@ and 'hexadecimal' can be used as well).\n"))
               (write-file file port #:select? select?)
               (force-output port)
               (get-hash))
-            (call-with-input-file file port-sha256))))
+            (match file
+              ("-" (port-sha256 (current-input-port)))
+              (_   (call-with-input-file file port-sha256))))))
 
     (match args
       ((file)

tests/guix-hash.sh

@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2013, 2014, 2016 Ludovic Courtès <ludo@gnu.org>
 # Copyright © 2016 Jan Nieuwenhuizen <janneke@gnu.org>
 #
 # This file is part of GNU Guix.
@@ -27,6 +27,7 @@ tmpdir="guix-hash-$$"
 trap 'rm -rf "$tmpdir"' EXIT
 
 test `guix hash /dev/null` = 0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73
+test `echo -n | guix hash -` = 0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73
 test `guix hash -f nix-base32 /dev/null` = 0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73
 test `guix hash -f hex /dev/null` = e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
 test `guix hash -f base32 /dev/null` = 4oymiquy7qobjgx36tejs35zeqt24qpemsnzgtfeswmrw6csxbkq