builtin/merge-tree: fix leaking -X strategy options

The `-X` switch for git-merge-tree(1) will push each option into a local
`xopts` vector that we then end up parsing. The vector never gets freed
though, causing a memory leak. Plug it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4301-merge-tree-write-tree.sh

@@ -2,6 +2,7 @@
 
 test_description='git merge-tree --write-tree'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 # This test is ort-specific