doc: describe Git bundle format

The bundle format was not documented. Describe the format with ABNF and
explain the meaning of each part.

Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt
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+= Git bundle v2 format
+
+The Git bundle format is a format that represents both refs and Git objects.
+
+== Format
+
+We will use ABNF notation to define the Git bundle format. See
+protocol-common.txt for the details.
+
+----
+bundle    = signature *prerequisite *reference LF pack
+signature = "# v2 git bundle" LF
+
+prerequisite = "-" obj-id SP comment LF
+comment      = *CHAR
+reference    = obj-id SP refname LF
+
+pack         = ... ; packfile
+----
+
+== Semantics
+
+A Git bundle consists of three parts.
+
+* "Prerequisites" lists the objects that are NOT included in the bundle and the
+  reader of the bundle MUST already have, in order to use the data in the
+  bundle. The objects stored in the bundle may refer to prerequisite objects and
+  anything reachable from them (e.g. a tree object in the bundle can reference
+  a blob that is reachable from a prerequisite) and/or expressed as a delta
+  against prerequisite objects.
+
+* "References" record the tips of the history graph, iow, what the reader of the
+  bundle CAN "git fetch" from it.
+
+* "Pack" is the pack data stream "git fetch" would send, if you fetch from a
+  repository that has the references recorded in the "References" above into a
+  repository that has references pointing at the objects listed in
+  "Prerequisites" above.
+
+In the bundle format, there can be a comment following a prerequisite obj-id.
+This is a comment and it has no specific meaning. The writer of the bundle MAY
+put any string here. The reader of the bundle MUST ignore the comment.
+
+=== Note on the shallow clone and a Git bundle
+
+Note that the prerequisites does not represent a shallow-clone boundary. The
+semantics of the prerequisites and the shallow-clone boundaries are different,
+and the Git bundle v2 format cannot represent a shallow clone repository.