show-index: the short help should say the command reads from its input

The short help text given by "git show-index -h" says

    $ git show-index -h
    usage: git show-index [--object-format=<hash-algorithm>]

        --[no-]object-format <hash-algorithm>
                              specify the hash algorithm to use

The command takes a pack .idx file from its standard input.  The
user has to _know_ this, as there is no indication from this output.

Give a hint that the data to work on is fed from its standard input.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

Diff for: Documentation/git-show-index.txt

@@ -9,7 +9,7 @@ git-show-index - Show packed archive index
 SYNOPSIS
 --------
 [verse]
-'git show-index' [--object-format=<hash-algorithm>]
+'git show-index' [--object-format=<hash-algorithm>] < <pack-idx-file>
 
 
 DESCRIPTION

Diff for: builtin/show-index.c

@@ -9,7 +9,7 @@
 #include "parse-options.h"
 
 static const char *const show_index_usage[] = {
-	"git show-index [--object-format=<hash-algorithm>]",
+	"git show-index [--object-format=<hash-algorithm>] < <pack-idx-file>",
 	NULL
 };