rebase-i: work around Windows CRLF line endings

rebase-i: work around Windows CRLF line endings

Editors on Windows can and do save text files with CRLF line
endings, which is the convention on the platform.  We are seeing
reports that the "read" command in a port of bash to the environment
however does not strip the CRLF at the end, not adjusting for the
same convention on the platform.

This breaks the recently added sanity checks for the insn sheet fed
to "rebase -i"; instead of an empty line (hence nothing in $command),
the script was getting a lone CR in there.

Special case a lone CR and treat it the same way as an empty line to
work this around.

This patch (also) passes the test with Git for Windows, where the
issue was seen first.

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