autoconf: GIT_CONF_APPEND_LINE: change signature

autoconf: GIT_CONF_APPEND_LINE: change signature

Change one-argument GIT_CONF_APPEND_LINE([VAR=VAL]) to
GIT_CONF_APPEND_LINE([VAR], [VAL]), that is more similar to the usual
AC_SUBST macro; this is only a preparatory change in view of future
refactorings.

No semantic change is intended.  In fact, the generated configure file
doesn't change at all.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>