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>
diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 6ed610a..0387b40 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -391,9 +391,6 @@
 	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 @@
 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