Merge branch 'dl/rebase-i-keep-base'

"git rebase --keep-base <upstream>" tries to find the original base
of the topic being rebased and rebase on top of that same base,
which is useful when running the "git rebase -i" (and its limited
variant "git rebase -x").

The command also has learned to fast-forward in more cases where it
can instead of replaying to recreate identical commits.

* dl/rebase-i-keep-base:
  rebase: teach rebase --keep-base
  rebase tests: test linear branch topology
  rebase: fast-forward --fork-point in more cases
  rebase: fast-forward --onto in more cases
  rebase: refactor can_fast_forward into goto tower
  t3432: test for --no-ff's interaction with fast-forward
  t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests
  t3432: test rebase fast-forward behavior
  t3431: add rebase --fork-point tests

1 files changed, 27 insertions, 3 deletions

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 7ecf766077..639a4179d1 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -8,8 +8,8 @@ git-rebase - Reapply commits on top of another base tip
 SYNOPSIS
 --------
 [verse]
-'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
-	[<upstream> [<branch>]]
+'git rebase' [-i | --interactive] [<options>] [--exec <cmd>]
+	[--onto <newbase> | --keep-base] [<upstream> [<branch>]]
 'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
 	--root [<branch>]
 'git rebase' (--continue | --skip | --abort | --quit | --edit-todo | --show-current-patch)
@@ -217,6 +217,24 @@ As a special case, you may use "A\...B" as a shortcut for the
 merge base of A and B if there is exactly one merge base. You can
 leave out at most one of A and B, in which case it defaults to HEAD.
 
+--keep-base::
+	Set the starting point at which to create the new commits to the
+	merge base of <upstream> <branch>. Running
+	'git rebase --keep-base <upstream> <branch>' is equivalent to
+	running 'git rebase --onto <upstream>... <upstream>'.
++
+This option is useful in the case where one is developing a feature on
+top of an upstream branch. While the feature is being worked on, the
+upstream branch may advance and it may not be the best idea to keep
+rebasing on top of the upstream but to keep the base commit as-is.
++
+Although both this option and --fork-point find the merge base between
+<upstream> and <branch>, this option uses the merge base as the _starting
+point_ on which new commits will be created, whereas --fork-point uses
+the merge base to determine the _set of commits_ which will be rebased.
++
+See also INCOMPATIBLE OPTIONS below.
+
 <upstream>::
 	Upstream branch to compare against.  May be any valid commit,
 	not just an existing branch name. Defaults to the configured
@@ -369,6 +387,10 @@ ends up being empty, the <upstream> will be used as a fallback.
 +
 If either <upstream> or --root is given on the command line, then the
 default is `--no-fork-point`, otherwise the default is `--fork-point`.
++
+If your branch was based on <upstream> but <upstream> was rewound and
+your branch contains commits which were dropped, this option can be used
+with `--keep-base` in order to drop those commits from your branch.
 
 --ignore-whitespace::
 --whitespace=<option>::
@@ -543,6 +565,8 @@ In addition, the following pairs of options are incompatible:
  * --preserve-merges and --interactive
  * --preserve-merges and --signoff
  * --preserve-merges and --rebase-merges
+ * --keep-base and --onto
+ * --keep-base and --root
 
 BEHAVIORAL DIFFERENCES
 -----------------------
@@ -869,7 +893,7 @@ NOTE: While an "easy case recovery" sometimes appears to be successful
       --interactive` will be **resurrected**!
 
 The idea is to manually tell 'git rebase' "where the old 'subsystem'
-ended and your 'topic' began", that is, what the old merge-base
+ended and your 'topic' began", that is, what the old merge base
 between them was.  You will have to find a way to name the last commit
 of the old 'subsystem', for example: