[PATCH] pack-format.txt: more details on pack file format

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

The current document mentions OBJ_* constants without their actual
values. A git developer would know these are from cache.h but that's
not very friendly to a person who wants to read this file to implement
a pack file parser.

Similarly, the deltified representation is not documented at all (the
"document" is basically patch-delta.c). Translate that C code in
English.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 I noticed that these object type values are not documented in
 pack-format.txt so here's my attempt to improve it.

 While at there, I also add some text about this obscure delta format.
 We occasionally have questions about this on the mailing list if I
 remember correctly.

 Documentation/technical/pack-format.txt | 41 +++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 8e5bf60be3..2c7d5c0e74 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -36,6 +36,47 @@ Git pack format
 
   - The trailer records 20-byte SHA-1 checksum of all of the above.
 
+Valid object types are:
+
+- OBJ_COMMIT (1)
+- OBJ_TREE (2)
+- OBJ_BLOB (3)
+- OBJ_TAG (4)
+- OBJ_OFS_DELTA (6)
+- OBJ_REF_DELTA (7)
+
+Type 5 is reserved for future expansion.
+
+Deltified representation is a sequence of one byte command optionally
+followed by more data for the command. The following commands are
+recognized:
+
+- If bit 7 is set, the remaining bits in the command byte specifies
+  how to extract copy offset and size to copy. The following must be
+  evaluated in this exact order:
+  - If bit 0 is set, the following byte contains bits 0-7 of the copy
+    offset (this also resets all other bits in the copy offset to
+    zero).
+  - If bit 1 is set, the following byte contains bits 8-15 of the copy
+    offset.
+  - If bit 2 is set, the following byte contains bits 16-23 of the
+    copy offset.
+  - If bit 3 is set, the following byte contains bits 24-31 of the
+    copy offset.
+  - If bit 4 is set, the following byte contains bits 0-7 of the copy
+    size (this also resets all other bits in the copy size to zero_.
+  - If bit 5 is set, the following byte contains bits 8-15 of the copy
+    size.
+  - If bit 6 is set, the following byte contains bits 16-23 of the
+    copy size.
+
+  Copy size zero means 0x10000 bytes. The data from source object at
+  the given copy offset is copied back to the destination buffer.
+
+- If bit 7 is not set, it is the copy size in bytes. The following
+  bytes are copied to destination buffer
+- Command byte zero is reserved for future expansion.
+
 == Original (version 1) pack-*.idx files have the following format:
 
   - The header consists of 256 4-byte network byte order
-- 
2.17.0.705.g3525833791