format-patch: add '--base' option to record base tree info

Maintainers or third party testers may want to know the exact base tree
the patch series applies to. Teach git format-patch a '--base' option
to record the base tree info and append it at the end of the first
message (either the cover letter or the first patch in the series).

The base tree info consists of the "base commit", which is a well-known
commit that is part of the stable part of the project history everybody
else works off of, and zero or more "prerequisite patches", which are
well-known patches in flight that is not yet part of the "base commit"
that need to be applied on top of "base commit" in topological order
before the patches can be applied.

The "base commit" is shown as "base-commit: " followed by the 40-hex of
the commit object name.  A "prerequisite patch" is shown as
"prerequisite-patch-id: " followed by the 40-hex "patch id", which can
be obtained by passing the patch through the "git patch-id --stable"
command.

Imagine that on top of the public commit P, you applied well-known
patches X, Y and Z from somebody else, and then built your three-patch
series A, B, C, the history would be like:

---P---X---Y---Z---A---B---C

With "git format-patch --base=P -3 C" (or variants thereof, e.g. with
"--cover-letter" of using "Z..C" instead of "-3 C" to specify the
range), the base tree information block is shown at the end of the
first message the command outputs (either the first patch, or the
cover letter), like this:

base-commit: P
prerequisite-patch-id: X
prerequisite-patch-id: Y
prerequisite-patch-id: Z

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 54 insertions, 0 deletions

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 6821441d7d..1d790f1af5 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -265,6 +265,11 @@ you can use `--suffix=-patch` to get `0001-description-of-my-change-patch`.
   Output an all-zero hash in each patch's From header instead
   of the hash of the commit.
 
+--base=<commit>::
+	Record the base tree information to identify the state the
+	patch series applies to.  See the BASE TREE INFORMATION section
+	below for details.
+
 --root::
 	Treat the revision argument as a <revision range>, even if it
 	is just a single commit (that would normally be treated as a
@@ -520,6 +525,55 @@ This should help you to submit patches inline using KMail.
 5. Back in the compose window: add whatever other text you wish to the
    message, complete the addressing and subject fields, and press send.
 
+BASE TREE INFORMATION
+---------------------
+
+The base tree information block is used for maintainers or third party
+testers to know the exact state the patch series applies to. It consists
+of the 'base commit', which is a well-known commit that is part of the
+stable part of the project history everybody else works off of, and zero
+or more 'prerequisite patches', which are well-known patches in flight
+that is not yet part of the 'base commit' that need to be applied on top
+of 'base commit' in topological order before the patches can be applied.
+
+The 'base commit' is shown as "base-commit: " followed by the 40-hex of
+the commit object name.  A 'prerequisite patch' is shown as
+"prerequisite-patch-id: " followed by the 40-hex 'patch id', which can
+be obtained by passing the patch through the `git patch-id --stable`
+command.
+
+Imagine that on top of the public commit P, you applied well-known
+patches X, Y and Z from somebody else, and then built your three-patch
+series A, B, C, the history would be like:
+
+................................................
+---P---X---Y---Z---A---B---C
+................................................
+
+With `git format-patch --base=P -3 C` (or variants thereof, e.g. with
+`--cover-letter` of using `Z..C` instead of `-3 C` to specify the
+range), the base tree information block is shown at the end of the
+first message the command outputs (either the first patch, or the
+cover letter), like this:
+
+------------
+base-commit: P
+prerequisite-patch-id: X
+prerequisite-patch-id: Y
+prerequisite-patch-id: Z
+------------
+
+For non-linear topology, such as
+
+................................................
+---P---X---A---M---C
+    \         /
+     Y---Z---B
+................................................
+
+You can also use `git format-patch --base=P -3 C` to generate patches
+for A, B and C, and the identifiers for P, X, Y, Z are appended at the
+end of the first message.
 
 EXAMPLES
 --------