Add a new test for using a custom merge strategy

Testing is done by creating a simple git-merge-theirs strategy which is
the opposite of ours. Using this in real merges is not recommended but
it's perfect for our testing needs.
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7606-merge-custom.sh

+#!/bin/sh
+
+test_description='git-merge
+
+Testing a custom strategy.'
+
+. ./test-lib.sh
+
+cat >git-merge-theirs <<EOF
+#!$SHELL_PATH
+eval git read-tree --reset -u \\\$\$#
+EOF
+chmod +x git-merge-theirs
+PATH=.:$PATH
+export PATH
+
+test_expect_success 'setup' '
+	echo c0 >c0.c &&
+	git add c0.c &&
+	git commit -m c0 &&
+	git tag c0 &&
+	echo c1 >c1.c &&
+	git add c1.c &&
+	git commit -m c1 &&
+	git tag c1 &&
+	git reset --hard c0 &&
+	echo c2 >c2.c &&
+	git add c2.c &&
+	git commit -m c2 &&
+	git tag c2
+'
+
+test_expect_success 'merge c2 with a custom strategy' '
+	git reset --hard c1 &&
+	git merge -s theirs c2 &&
+	test "$(git rev-parse c1)" != "$(git rev-parse HEAD)" &&
+	test "$(git rev-parse c1)" = "$(git rev-parse HEAD^1)" &&
+	test "$(git rev-parse c2)" = "$(git rev-parse HEAD^2)" &&
+	test "$(git rev-parse c2^{tree})" = "$(git rev-parse HEAD^{tree})" &&
+	git diff --exit-code &&
+	test -f c0.c &&
+	test ! -f c1.c &&
+	test -f c2.c
+'
+
+test_done