rebase: add documentation and test for --no-rebase-merges

As far as I can tell, --no-rebase-merges has always worked, but has
never been documented. It is especially important to document it before
a rebase.rebaseMerges option is introduced so that users know how to
override the config option on the command line. It's also important to
clarify that --rebase-merges without an argument is not the same as
--no-rebase-merges and not passing --rebase-merges is not the same as
passing --rebase-merges=no-rebase-cousins.

A test case is necessary to make sure that --no-rebase-merges keeps
working after its code is refactored in the following patches of this
series. The test case is a little contrived: It's unlikely that a user
would type both --rebase-merges and --no-rebase-merges at the same time.
However, if an alias is defined which includes --rebase-merges, the user
might decide to add --no-rebase-merges to countermand that part of the
alias but leave alone other flags set by the alias.

Signed-off-by: Alex Henrie <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: alexhenrie

Documentation/git-rebase.txt

@@ -529,20 +529,24 @@ See also INCOMPATIBLE OPTIONS below.
 
 -r::
 --rebase-merges[=(rebase-cousins|no-rebase-cousins)]::
+--no-rebase-merges::
 	By default, a rebase will simply drop merge commits from the todo
 	list, and put the rebased commits into a single, linear branch.
 	With `--rebase-merges`, the rebase will instead try to preserve
 	the branching structure within the commits that are to be rebased,
 	by recreating the merge commits. Any resolved merge conflicts or
 	manual amendments in these merge commits will have to be
-	resolved/re-applied manually.
+	resolved/re-applied manually. `--no-rebase-merges` can be used to
+	countermand a previous `--rebase-merges`.
 +
-By default, or when `no-rebase-cousins` was specified, commits which do not
-have `<upstream>` as direct ancestor will keep their original branch point,
-i.e. commits that would be excluded by linkgit:git-log[1]'s
-`--ancestry-path` option will keep their original ancestry by default. If
-the `rebase-cousins` mode is turned on, such commits are instead rebased
-onto `<upstream>` (or `<onto>`, if specified).
+When rebasing merges, there are two modes: `rebase-cousins` and
+`no-rebase-cousins`. If the mode is not specified, it defaults to
+`no-rebase-cousins`. In `no-rebase-cousins` mode, commits which do not have
+`<upstream>` as direct ancestor will keep their original branch point, i.e.
+commits that would be excluded by linkgit:git-log[1]'s `--ancestry-path`
+option will keep their original ancestry by default. In `rebase-cousins` mode,
+such commits are instead rebased onto `<upstream>` (or `<onto>`, if
+specified).
 +
 It is currently only possible to recreate the merge commits using the
 `ort` merge strategy; different merge strategies can be used only via

t/t3430-rebase-merges.sh

@@ -250,6 +250,16 @@ test_expect_success 'with a branch tip that was cherry-picked already' '
 	EOF
 '
 
+test_expect_success '--no-rebase-merges countermands --rebase-merges' '
+	git checkout -b no-rebase-merges E &&
+	git rebase --rebase-merges --no-rebase-merges C &&
+	test_cmp_graph C.. <<-\EOF
+	* B
+	* D
+	o C
+	EOF
+'
+
 test_expect_success 'do not rebase cousins unless asked for' '
 	git checkout -b cousins main &&
 	before="$(git rev-parse --verify HEAD)" &&