rebase -i: clean error message for --continue after failed exec

After an "exec false" stops the rebase and gives the control back to
the user, if changes are added to the index, "rebase --continue" fails
with this message, which may technically be correct, but does not point
at the real problem:

.../git-rebase--interactive: line 774: .../.git/rebase-merge/author-script: No such file or directory

We could try auto-amending HEAD, but this goes against the logic of
.git/rebase-merge/author-script (see also the testcase 'auto-amend only
edited commits after "edit"' in t3404-rebase-interactive.sh) to
auto-amend something the user hasn't explicitely asked to edit.

Instead of doing anything automatically, detect the situation and give a
clean error message. While we're there, also clarify the error message in
case '. "$author_script"' fails, which now corresponds to really weird
senario where the author script exists but can't be read.
Test-case-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -647,8 +647,24 @@ continue)
 	then
 		: Nothing to commit -- skip this
 	else
+		if ! test -f "$author_script"
+		then
+			die "You have staged changes in your working tree. If these changes are meant to be
+squashed into the previous commit, run:
+
+  git commit --amend
+
+If they are meant to go into a new commit, run:
+
+  git commit
+
+In both case, once you're done, continue with:
+
+  git rebase --continue
+"
+		fi
 		. "$author_script" ||
-			die "Cannot find the author identity"
+			die "Error trying to find the author identity to amend commit"
 		current_head=
 		if test -f "$amend"
 		then

t/t3404-rebase-interactive.sh

@@ -527,6 +527,20 @@ test_expect_success 'auto-amend only edited commits after "edit"' '
 	git rebase --abort
 '
 
+test_expect_success 'clean error after failed "exec"' '
+	test_tick &&
+	test_when_finished "git rebase --abort || :" &&
+	(
+		FAKE_LINES="1 exec_false" &&
+		export FAKE_LINES &&
+		test_must_fail git rebase -i HEAD^
+	) &&
+	echo "edited again" > file7 &&
+	git add file7 &&
+	test_must_fail git rebase --continue 2>error &&
+	grep "You have staged changes in your working tree." error
+'
+
 test_expect_success 'rebase a detached HEAD' '
 	grandparent=$(git rev-parse HEAD~2) &&
 	git checkout $(git rev-parse HEAD) &&