combine-diff.c: type sanity.

In diff_tree_combined(), show_log_first boolean is initialized with
rev->loginfo (pointer to a string); the intention is that if we have
some string to be emitted we would want to remember that fact.  Picky
compilers are offended by this, so make the expression a bit type-safer.

Signed-off-by: Junio C Hamano <junkio@cox.net>

combine-diff.c

@@ -841,7 +841,7 @@ void diff_tree_combined(const unsigned char *sha1,
 	diffopts.output_format = DIFF_FORMAT_NO_OUTPUT;
 	diffopts.recursive = 1;
 
-	show_log_first = rev->loginfo;
+	show_log_first = !!rev->loginfo;
 	needsep = 0;
 	/* find set of paths that everybody touches */
 	for (i = 0; i < num_parent; i++) {