rev-list: fix --reverse interaction with --parents

--reverse did not interact well with --parents, as the included test
case shows: in a history like

  A--B.
   \   \
    `C--M--D

the command

  git rev-list --reverse --parents --full-history HEAD

erroneously lists D as having no parents at all.  (Without --reverse,
it correctly lists M.)

This is caused by the machinery driving --reverse: it first grabs all
commits through the normal routines, then runs them through the same
routines again, effectively simplifying them twice.

Fix this by moving the --reverse one level up, into get_revision().
This way we can cleanly grab all commits via the normal calls, then
just pop them off the list one by one without interfering with
get_revision_internal().

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6013-rev-list-reverse-parents.sh

@@ -0,0 +1,42 @@
+#!/bin/sh
+
+test_description='--reverse combines with --parents'
+
+. ./test-lib.sh
+
+
+commit () {
+	test_tick &&
+	echo $1 > foo &&
+	git add foo &&
+	git commit -m "$1"
+}
+
+test_expect_success 'set up --reverse example' '
+	commit one &&
+	git tag root &&
+	commit two &&
+	git checkout -b side HEAD^ &&
+	commit three &&
+	git checkout master &&
+	git merge -s ours side &&
+	commit five
+	'
+
+test_expect_success '--reverse --parents --full-history combines correctly' '
+	git rev-list --parents --full-history master -- foo |
+		tac > expected &&
+	git rev-list --reverse --parents --full-history master -- foo \
+		> actual &&
+	test_cmp actual expected
+	'
+
+test_expect_success '--boundary does too' '
+	git rev-list --boundary --parents --full-history master ^root -- foo |
+		tac > expected &&
+	git rev-list --boundary --reverse --parents --full-history \
+		master ^root -- foo > actual &&
+	test_cmp actual expected
+	'
+
+test_done