Add a testcase for "git mv -f" on untracked files.

This currently fails with:
git: builtin-mv.c:217: cmd_mv: Assertion `pos >= 0' failed.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7001-mv.sh

@@ -58,6 +58,14 @@ test_expect_success \
      test ! -f path0/untracked1 &&
      test ! -f path0/untracked2'
 
+test_expect_failure \
+    'checking -f on untracked file with existing target' \
+    'touch path0/untracked1 &&
+     git mv -f untracked1 path0
+     test ! -f .git/index.lock &&
+     test -f untracked1 &&
+     test -f path0/untracked1'
+
 # clean up the mess in case bad things happen
 rm -f idontexist untracked1 untracked2 \
      path0/idontexist path0/untracked1 path0/untracked2 \