git mv foo FOO ; git mv foo bar gave an assert

git mv foo FOO ; git mv foo bar gave an assert

The following sequence, on a case-insensitive file system,
(strictly speeking with core.ignorecase=true)
leads to an assertion failure and leaves .git/index.lock behind.

git init
echo foo >foo
git add foo
git mv foo FOO
git mv foo bar

This regression was introduced in Commit 9b906af657,
"git-mv: improve error message for conflicted file"

The bugfix is to change the "file exist case-insensitive in the index"
into the correct "file exist (case-sensitive) in the index".

This avoids the "assert" later in the code and keeps setting up the
"ce" pointer for ce_stage(ce) done in the next else if.

This fixes
https://github.com/git-for-windows/git/issues/2920

Reported-By: Dan Moseley <Dan.Moseley@microsoft.com>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>