rebase: decouple --exec from --interactive

In the later steps of preparing a patch series I do not want to
edit or reorder the patches any more, but just make sure the
test suite passes after each patch and also to fix breakage
right there if some of the steps fail.  I could run

    EDITOR=true git rebase -i <anchor> -x "make test"

but it would be simpler if it can be spelled like so:

    git rebase <anchor> -x "make test"

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -391,9 +391,6 @@ idea unless you know what you are doing (see BUGS below).
 	final history. <cmd> will be interpreted as one or more shell
 	commands.
 +
-This option can only be used with the `--interactive` option
-(see INTERACTIVE MODE below).
-+
 You may execute several commands by either using one instance of `--exec`
 with several commands:
 +
@@ -406,6 +403,9 @@ or by giving more than one `--exec`:
 If `--autosquash` is used, "exec" lines will not be appended for
 the intermediate commits, and will only appear at the end of each
 squash/fixup series.
++
+This uses the `--interactive` machinery internally, but it can be run
+without an explicit `--interactive`.
 
 --root::
 	Rebase all commits reachable from <branch>, instead of

git-rebase.sh

@@ -248,6 +248,7 @@ do
 		;;
 	--exec=*)
 		cmd="${cmd}exec ${1#--exec=}${LF}"
+		test -z "$interactive_rebase" && interactive_rebase=implied
 		;;
 	--interactive)
 		interactive_rebase=explicit
@@ -348,12 +349,6 @@ do
 done
 test $# -gt 2 && usage
 
-if test -n "$cmd" &&
-   test "$interactive_rebase" != explicit
-then
-	die "$(gettext "The --exec option must be used with the --interactive option")"
-fi
-
 if test -n "$action"
 then
 	test -z "$in_progress" && die "$(gettext "No rebase in progress?")"

t/t3404-rebase-interactive.sh

@@ -876,16 +876,15 @@ test_expect_success 'rebase -ix with --autosquash' '
 	test_cmp expected actual
 '
 
-
-test_expect_success 'rebase --exec without -i shows error message' '
+test_expect_success 'rebase --exec works without -i ' '
 	git reset --hard execute &&
-	set_fake_editor &&
-	test_must_fail git rebase --exec "git show HEAD" HEAD~2 2>actual &&
-	echo "The --exec option must be used with the --interactive option" >expected &&
-	test_i18ncmp expected actual
+	rm -rf exec_output &&
+	EDITOR="echo >invoked_editor" git rebase --exec "echo a line >>exec_output"  HEAD~2 2>actual &&
+	test_i18ngrep  "Successfully rebased and updated" actual &&
+	test_line_count = 2 exec_output &&
+	test_path_is_missing invoked_editor
 '
 
-
 test_expect_success 'rebase -i --exec without <CMD>' '
 	git reset --hard execute &&
 	set_fake_editor &&