convert: rename the "eol" global variable to "core_eol"

convert: rename the "eol" global variable to "core_eol"

Yes, it is clear that "eol" wants to mean some sort of end-of-line thing,
but as the name of a global variable, it is way too short to describe what
kind of end-of-line thing it wants to represent. Besides, there are many
codepaths that want to use their own local "char *eol" variable to point
at the end of the current line they are processing.

This global variable holds what we read from core.eol configuration
variable. Name it as such.

Signed-off-by: Junio C Hamano <gitster@pobox.com>