Re: [PATCH 1/2] docs: document SHA-256 pack and indices

> Now that we have SHA-256 support for packs and indices, let's document
> that in SHA-256 repositories, we use SHA-256 instead of SHA-1 for object
> names and checksums.  Instead of duplicating this information throughout
> the document, let's just document that in SHA-1 repositories, we use
> SHA-1 for these purposes, and in SHA-256 repositories, we use SHA-256.
> 
> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
> ---
>  Documentation/technical/pack-format.txt | 36 ++++++++++++++-----------
>  1 file changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
> index d3a142c652..f4c8d94f73 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
> @@ -1,6 +1,12 @@
>  Git pack format
>  ===============
>  
> +== Checksums and object IDs
> +
> +In a repository using the traditional SHA-1, pack checksums, index checksums,
> +and object IDs (object names) mentioned below are all computed using SHA-1.
> +Similarly, in SHA-256 repositories, these values are computed using SHA-256.
> +
>  == pack-*.pack files have the following format:
>  
>     - A header appears at the beginning and consists of the following:
> @@ -26,7 +32,7 @@ Git pack format
>  
>       (deltified representation)
>       n-byte type and length (3-bit type, (n-1)*7+4-bit length)
> -     20-byte base object name if OBJ_REF_DELTA or a negative relative
> +     base object name if OBJ_REF_DELTA or a negative relative
>  	 offset from the delta object's position in the pack if this
>  	 is an OBJ_OFS_DELTA object
>       compressed delta data
> @@ -34,7 +40,7 @@ Git pack format
>       Observation: length of each object is encoded in a variable
>       length format and is not constrained to 32-bit or anything.
>  
> -  - The trailer records 20-byte SHA-1 checksum of all of the above.
> +  - The trailer records a pack checksum of all of the above.
>  
>  === Object types
>  
> @@ -58,8 +64,8 @@ ofs-delta and ref-delta, which is only valid in a pack file.
>  
>  Both ofs-delta and ref-delta store the "delta" to be applied to
>  another object (called 'base object') to reconstruct the object. The
> -difference between them is, ref-delta directly encodes 20-byte base
> -object name. If the base object is in the same pack, ofs-delta encodes
> +difference between them is, ref-delta directly encodes base object
> +name. If the base object is in the same pack, ofs-delta encodes
>  the offset of the base object in the pack instead.
>  
>  The base object could also be deltified if it's in the same pack.
> @@ -143,14 +149,14 @@ This is the instruction reserved for future expansion.
>      object is stored in the packfile as the offset from the
>      beginning.
>  
> -    20-byte object name.
> +    one object name of the appropriate size.
>  
>    - The file is concluded with a trailer:
>  
> -    A copy of the 20-byte SHA-1 checksum at the end of
> -    corresponding packfile.
> +    A copy of the pack checksum at the end of the corresponding
> +    packfile.
>  
> -    20-byte SHA-1-checksum of all of the above.
> +    Index checksum of all of the above.
>  
>  Pack Idx file:
>  
> @@ -198,7 +204,7 @@ Pack file entry: <+
>          If it is not DELTA, then deflated bytes (the size above
>  		is the size before compression).
>  	If it is REF_DELTA, then
> -	  20-byte base object name SHA-1 (the size above is the
> +	  base object name (the size above is the
>  		size of the delta data that follows).
>            delta data, deflated.
>  	If it is OFS_DELTA, then
> @@ -227,9 +233,9 @@ Pack file entry: <+
>  
>    - A 256-entry fan-out table just like v1.
>  
> -  - A table of sorted 20-byte SHA-1 object names.  These are
> -    packed together without offset values to reduce the cache
> -    footprint of the binary search for a specific object name.
> +  - A table of sorted object names.  These are packed together
> +    without offset values to reduce the cache footprint of the
> +    binary search for a specific object name.
>  
>    - A table of 4-byte CRC32 values of the packed object data.
>      This is new in v2 so compressed data can be copied directly
> @@ -248,10 +254,10 @@ Pack file entry: <+
>  
>    - The same trailer as a v1 pack file:
>  
> -    A copy of the 20-byte SHA-1 checksum at the end of
> +    A copy of the pack checksum at the end of
>      corresponding packfile.
>  
> -    20-byte SHA-1-checksum of all of the above.
> +    Index checksum of all of the above.
>  
>  == multi-pack-index (MIDX) files have the following format:
>  
> @@ -329,4 +335,4 @@ CHUNK DATA:
>  
>  TRAILER:
>  
> -	20-byte SHA1-checksum of the above contents.
> +	Index checksum of the above contents.