[PATCH] docs: document zero bits in index "mode"

[Glen Choo <chooglen@google.com>]

Documentation/gitformat-index.txt describes the "mode" as 32 bits, but
only documents 16 bits. Document the missing 16 bits and specify that
'unused' bits must be zero.

Signed-off-by: Glen Choo <chooglen@google.com>
---
>                 The existing explanation starts with "32-bit mode,
> split into (high to low bits)", followed by "4-bit object type", as
> if the "4-bit object type" occupies bits 29-32, which is not quite
> what we want to say.

If I am understanding you correctly (which I'm not sure, since I am
honestly clueless about bit numbering and big endianness and whatnot),
you're saying that highest 16 bits are zero, not the lowest? If so, then
I genuinely made that mistake, hah.

(We're storing 16 bits as bigendian 32 bits, so they would occupy the
lower 16 bits, right..?)

Perhaps something like this would be better. I took the "unused, must be
zero" phrasing from elsewhere in the doc. And if I am completely
off-base, feel free to patch it without waiting for my reroll if you
think that's easier.

 Documentation/gitformat-index.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/gitformat-index.txt b/Documentation/gitformat-index.txt
index 015cb21bdc..0773e5c380 100644
--- a/Documentation/gitformat-index.txt
+++ b/Documentation/gitformat-index.txt
@@ -83,11 +83,13 @@ Git index format
 
   32-bit mode, split into (high to low bits)
 
+    16-bit unused, must be zero
+
     4-bit object type
       valid values in binary are 1000 (regular file), 1010 (symbolic link)
       and 1110 (gitlink)
 
-    3-bit unused
+    3-bit unused, must be zero
 
     9-bit unix permission. Only 0755 and 0644 are valid for regular files.
     Symbolic links and gitlinks have value 0 in this field.
-- 
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