rebase -i: introduce --rebase-merges=[no-]rebase-cousins

When running `git rebase --rebase-merges` non-interactively with an
ancestor of HEAD as <upstream> (or leaving the todo list unmodified),
we would ideally recreate the exact same commits as before the rebase.

However, if there are commits in the commit range <upstream>.. that do not
have <upstream> as direct ancestor (i.e. if `git log <upstream>..` would
show commits that are omitted by `git log --ancestry-path <upstream>..`),
this is currently not the case: we would turn them into commits that have
<upstream> as direct ancestor.

Let's illustrate that with a diagram:

        C
      /   \
A - B - E - F
  \   /
    D

Currently, after running `git rebase -i --rebase-merges B`, the new branch
structure would be (pay particular attention to the commit `D`):

       --- C' --
      /         \
A - B ------ E' - F'
      \    /
        D'

This is not really preserving the branch topology from before! The
reason is that the commit `D` does not have `B` as ancestor, and
therefore it gets rebased onto `B`.

This is unintuitive behavior. Even worse, when recreating branch
structure, most use cases would appear to want cousins *not* to be
rebased onto the new base commit. For example, Git for Windows (the
heaviest user of the Git garden shears, which served as the blueprint
for --rebase-merges) frequently merges branches from `next` early, and
these branches certainly do *not* want to be rebased. In the example
above, the desired outcome would look like this:

       --- C' --
      /         \
A - B ------ E' - F'
  \        /
   -- D' --

Let's introduce the term "cousins" for such commits ("D" in the
example), and let's not rebase them by default. For hypothetical
use cases where cousins *do* need to be rebased, `git rebase
--rebase=merges=rebase-cousins` needs to be used.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/builtin/rebase--helper.c b/builtin/rebase--helper.c
index 781782e727..f7c2a5fdc8 100644
--- a/builtin/rebase--helper.c
+++ b/builtin/rebase--helper.c
@@ -13,7 +13,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 {
 	struct replay_opts opts = REPLAY_OPTS_INIT;
 	unsigned flags = 0, keep_empty = 0, rebase_merges = 0;
-	int abbreviate_commands = 0;
+	int abbreviate_commands = 0, rebase_cousins = -1;
 	enum {
 		CONTINUE = 1, ABORT, MAKE_SCRIPT, SHORTEN_OIDS, EXPAND_OIDS,
 		CHECK_TODO_LIST, SKIP_UNNECESSARY_PICKS, REARRANGE_SQUASH,
@@ -25,6 +25,8 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "allow-empty-message", &opts.allow_empty_message,
 			N_("allow commits with empty messages")),
 		OPT_BOOL(0, "rebase-merges", &rebase_merges, N_("rebase merge commits")),
+		OPT_BOOL(0, "rebase-cousins", &rebase_cousins,
+			 N_("keep original branch points of cousins")),
 		OPT_CMDMODE(0, "continue", &command, N_("continue rebase"),
 				CONTINUE),
 		OPT_CMDMODE(0, "abort", &command, N_("abort rebase"),
@@ -59,8 +61,13 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	flags |= keep_empty ? TODO_LIST_KEEP_EMPTY : 0;
 	flags |= abbreviate_commands ? TODO_LIST_ABBREVIATE_CMDS : 0;
 	flags |= rebase_merges ? TODO_LIST_REBASE_MERGES : 0;
+	flags |= rebase_cousins > 0 ? TODO_LIST_REBASE_COUSINS : 0;
 	flags |= command == SHORTEN_OIDS ? TODO_LIST_SHORTEN_IDS : 0;
 
+	if (rebase_cousins >= 0 && !rebase_merges)
+		warning(_("--[no-]rebase-cousins has no effect without "
+			  "--rebase-merges"));
+
 	if (command == CONTINUE && argc == 1)
 		return !!sequencer_continue(&opts);
 	if (command == ABORT && argc == 1)