rebase: fix format.useAutoBase breakage

With `format.useAutoBase = true`, running rebase resulted in an
error:

	fatal: failed to get upstream, if you want to record base commit automatically,
	please use git branch --set-upstream-to to track a remote branch.
	Or you could specify base commit by --base=<base-commit-id> manually
	error:
	git encountered an error while preparing the patches to replay
	these revisions:

	    ede2467cdedc63784887b587a61c36b7850ebfac..d8f581194799ae29bf5fa72a98cbae98a1198b12

	As a result, git cannot rebase them.

Fix this by always passing `--no-base` to format-patch from rebase so
that the effect of `format.useAutoBase` is negated.

Reported-by: Christian Biesinger <cbiesinger@google.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1012,7 +1012,8 @@ static int run_am(struct rebase_options *opts)
 	argv_array_pushl(&format_patch.args, "format-patch", "-k", "--stdout",
 			 "--full-index", "--cherry-pick", "--right-only",
 			 "--src-prefix=a/", "--dst-prefix=b/", "--no-renames",
-			 "--no-cover-letter", "--pretty=mboxrd", "--topo-order", NULL);
+			 "--no-cover-letter", "--pretty=mboxrd", "--topo-order",
+			 "--no-base", NULL);
 	if (opts->git_format_patch_opt.len)
 		argv_array_split(&format_patch.args,
 				 opts->git_format_patch_opt.buf);

t/t3400-rebase.sh

@@ -159,7 +159,7 @@ test_expect_success 'fail when upstream arg is missing and not configured' '
 	test_must_fail git rebase
 '
 
-test_expect_failure 'rebase works with format.useAutoBase' '
+test_expect_success 'rebase works with format.useAutoBase' '
 	test_config format.useAutoBase true &&
 	git checkout topic &&
 	git rebase master