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

[Junio C Hamano <gitster@pobox.com>]

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>
---

 * I also found the option description somewhat funny in that

   (1) it makes it look like "--no-object-format sha256" is
   accepted, which is not a case, and

   (2) "git show-index --no-object-format" already is a curious
   thing to say; the command certainly needs to work in _some_
   format.

   But (2) is common to all the usual command line options to allow
   defeating another instance of the same option that is given
   positively previously on the command line (i.e. "git show-index
   --object-format=sha256 --no-object-format" should behave as if no
   object-format option was given), and (1) is shared by all the
   other options that allow such override.  So I'll let it pass, but
   if we really wanted to improve it, the fix should go into how the
   parse-options subsystem works.

 builtin/show-index.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git c/builtin/show-index.c w/builtin/show-index.c
index f164c01bbe..8678b741a4 100644
--- c/builtin/show-index.c
+++ w/builtin/show-index.c
@@ -7,7 +7,7 @@
 #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
 };