[BUG] - git-read-tree segfaults

Hi,

I was following the git core tutorial and git-read-tree got me segfault.
I accidentaly executed the git-read-tree without "-m -u" options and
got segfault.

I can reproduce with the latest git using following script.

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#!/bin/sh

GIT=$1

rm -rf crash; mkdir -p crash; cd crash
$GIT init
echo "xxx" > xxx
$GIT add xxx
$GIT commit -m "xxx"
$GIT checkout -b yyy
echo "yyy" > yyy
$GIT add yyy
$GIT commit -m "yyy"
echo "yyy1" >> yyy
$GIT commit -a -m "yyy1"
$GIT checkout master
echo "xxx1" >> xxx
$GIT commit -a -m "xxx1"
mb=$($GIT merge-base HEAD yyy)
$GIT read-tree $mb HEAD yyy
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regards,
jirka