git add -p: demonstrate failure when staging both mode and hunk

When trying to stage changes to file which has also pending `chmod +x`,
`git add -p` produces lots of 'Use of uninitialized value ...' warnings
and fails to do the job:

    $ echo content >> file
    $ chmod +x file
    $ git add -p
    diff --git a/file b/file
    index e69de29..d95f3ad
    --- a/file
    +++ b/file
    old mode 100644
    new mode 100755
    Stage mode change [y,n,q,a,d,/,j,J,g,?]? y
    @@ -0,0 +1 @@
    +content
    Stage this hunk [y,n,q,a,d,/,K,g,e,?]? y
    Use of uninitialized value $o_ofs in addition (+) at .../git-add--interactive line 776.
    Use of uninitialized value $ofs in numeric le (<=) at .../git-add--interactive line 806.
    Use of uninitialized value $o0_ofs in concatenation (.) or string at .../git-add--interactive line 830.
    Use of uninitialized value $n0_ofs in concatenation (.) or string at .../git-add--interactive line 830.
    Use of uninitialized value $o_ofs in addition (+) at .../git-add--interactive line 776.
    fatal: corrupt patch at line 5
    diff --git a/file b/file
    index e69de29..d95f3ad
    --- a/file
    +++ b/file
    @@ -,0 + @@
    +content
Signed-off-by: NKirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t3701-add-interactive.sh

@@ -163,6 +163,17 @@ test_expect_success FILEMODE 'stage mode but not hunk' '
 	git diff          file | grep "+content"
 '
 
+
+test_expect_failure FILEMODE 'stage mode and hunk' '
+	git reset --hard &&
+	echo content >>file &&
+	chmod +x file &&
+	printf "y\\ny\\n" | git add -p &&
+	git diff --cached file | grep "new mode" &&
+	git diff --cached file | grep "+content" &&
+	test -z "$(git diff file)"
+'
+
 # end of tests disabled when filemode is not usable
 
 # Write the patch file with a new line at the top and bottom