move locate_head() to remote.c

move locate_head() to remote.c

Move locate_head() to remote.c and rename it to guess_remote_head() to
more accurately reflect what it does. This is in preparation for being
able to call it from builtin-remote.c

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>