In order to fix the warning, we add a new "merge-file.h" header
containing the extern declaration of the merge_file() function,
and include the header in the source files that require the
declaration.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>