convert: tighten the safe autocrlf handling

convert: tighten the safe autocrlf handling

When a text file had been commited with CRLF and the file is commited
again, the CRLF are kept if .gitattributs has "text=auto".
This is done by analyzing the content of the blob stored in the index:
If a '\r' is found, Git assumes that the blob was commited with CRLF.

The simple search for a '\r' does not always work as expected:
A file is encoded in UTF-16 with CRLF and commited. Git treats it as binary.
Now the content is converted into UTF-8. At the next commit Git treats the
file as text, the CRLF should be converted into LF, but isn't.

Replace has_cr_in_index() with has_crlf_in_index(). When no '\r' is found,
0 is returned directly, this is the most common case.
If a '\r' is found, the content is analyzed more deeply.

Reported-By: Ashish Negi <ashishnegi33@gmail.com>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>