Merge branch 'jc/doc-submitting-patches-choice-of-base'

Extend the guidance to choose the base commit to build your work
on, and hint/nudge contributors to read others' changes.

* jc/doc-submitting-patches-choice-of-base:
  SubmittingPatchs: clarify choice of base and testing

Documentation/SubmittingPatches

@@ -19,8 +19,10 @@ change is relevant to.
   base your work on the tip of the topic.
 
 * A new feature should be based on `master` in general. If the new
-  feature depends on a topic that is in `seen`, but not in `master`,
+  feature depends on other topics that are in `next`, but not in
-  base your work on the tip of that topic.
+  `master`, fork a branch from the tip of `master`, merge these topics
+  to the branch, and work on that branch.  You can remind yourself of
+  how you prepared the base with `git log --first-parent master..`.
 
 * Corrections and enhancements to a topic not yet in `master` should
   be based on the tip of that topic. If the topic has not been merged
@@ -28,10 +30,10 @@ change is relevant to.
   into the series.
 
 * In the exceptional case that a new feature depends on several topics
-  not in `master`, start working on `next` or `seen` privately and send
+  not in `master`, start working on `next` or `seen` privately and
-  out patches for discussion. Before the final merge, you may have to
+  send out patches only for discussion. Once your new feature starts
-  wait until some of the dependent topics graduate to `master`, and
+  to stabilize, you would have to rebase it (see the "depends on other
-  rebase your work.
+  topics" above).
 
 * Some parts of the system have dedicated maintainers with their own
   repositories (see the section "Subsystems" below).  Changes to
@@ -71,8 +73,13 @@ Make sure that you have tests for the bug you are fixing.  See
 [[tests]]
 When adding a new feature, make sure that you have new tests to show
 the feature triggers the new behavior when it should, and to show the
-feature does not trigger when it shouldn't.  After any code change, make
+feature does not trigger when it shouldn't.  After any code change,
-sure that the entire test suite passes.
+make sure that the entire test suite passes.  When fixing a bug, make
+sure you have new tests that break if somebody else breaks what you
+fixed by accident to avoid regression.  Also, try merging your work to
+'next' and 'seen' and make sure the tests still pass; topics by others
+that are still in flight may have unexpected interactions with what
+you are trying to do in your topic.
 
 Pushing to a fork of https://github.com/git/git will use their CI
 integration to test your changes on Linux, Mac and Windows. See the
@@ -144,8 +151,21 @@ without external resources. Instead of giving a URL to a mailing list
 archive, summarize the relevant points of the discussion.
 
 [[commit-reference]]
-If you want to reference a previous commit in the history of a stable
+
-branch, use the format "abbreviated hash (subject, date)", like this:
+There are a few reasons why you may want to refer to another commit in
+the "more stable" part of the history (i.e. on branches like `maint`,
+`master`, and `next`):
+
+. A commit that introduced the root cause of a bug you are fixing.
+
+. A commit that introduced a feature that you are enhancing.
+
+. A commit that conflicts with your work when you made a trial merge
+  of your work into `next` and `seen` for testing.
+
+When you reference a commit on a more stable branch (like `master`,
+`maint` and `next`), use the format "abbreviated hash (subject,
+date)", like this:
 
 ....
 	Commit f86a374 (pack-bitmap.c: fix a memleak, 2015-03-30)
@@ -259,9 +279,11 @@ Please make sure your patch does not add commented out debugging code,
 or include any extra files which do not relate to what your patch
 is trying to achieve. Make sure to review
 your patch after generating it, to ensure accuracy.  Before
-sending out, please make sure it cleanly applies to the `master`
+sending out, please make sure it cleanly applies to the base you
-branch head.  If you are preparing a work based on "next" branch,
+have chosen in the "Decide what to base your work on" section,
-that is fine, but please mark it as such.
+and unless it targets the `master` branch (which is the default),
+mark your patches as such.
+
 
 [[send-patches]]
 === Sending your patches.
@@ -365,7 +387,10 @@ Security mailing list{security-ml-ref}.
 Send your patch with "To:" set to the mailing list, with "cc:" listing
 people who are involved in the area you are touching (the `git
 contacts` command in `contrib/contacts/` can help to
-identify them), to solicit comments and reviews.
+identify them), to solicit comments and reviews.  Also, when you made
+trial merges of your topic to `next` and `seen`, you may have noticed
+work by others conflicting with your changes.  There is a good possibility
+that these people may know the area you are touching well.
 
 :current-maintainer: footnote:[The current maintainer: gitster@pobox.com]
 :git-ml: footnote:[The mailing list: git@vger.kernel.org]