Add testcase for merging in a CRLF repo

Add testcase for merging in a CRLF repo

If you work on a repo with core.autocrlf == true, you would expect
every text file to have CRLF EOLs. However, if you by some operation,
get a conflict, then the conflicted file has LF EOLs.

Now, of course you'd go about resolving the files conflict, and then 'git
add <file>'. When you do that, you'll get the warning saying that LF will
be replaced by CRLF. Then you commit. The end result is that you have a
workingdir with a mix of LF and CRLF files, which after some more
operations may trigger a "whole file changed" diff, due to the workingdir
file now having LF EOLs.

An LF only conflict file results in the resolved file being in LF,
the commit is in LF and a warning saying that LF will be replaced
by CRLF, and the working dir ends up with a mix of CRLF and LF files.

Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>