Increase length of function name buffer

Increase length of function name buffer

In xemit.c:xdl_emit_diff() a buffer for showing the function name as
commentary is allocated; this buffer was 40 characters.  This is a bit
small; particularly for C++ function names where there is often an
identical prefix (like void LongNamespace::LongClassName) on multiple
functions, which makes the context the same everywhere.  In other words
the context is useless.  This patch increases that buffer to 80
characters - which may still not be enough, but is better

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>