revision.h: make show_early_output an extern which is defined in revision.c

The variable show_early_output is defined in revision.c and should be
declared extern in revision.h so that the linker does not complain
about multiply defined variables.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -119,7 +119,7 @@ struct rev_info {
 void read_revisions_from_stdin(struct rev_info *revs);
 
 typedef void (*show_early_output_fn_t)(struct rev_info *, struct commit_list *);
-volatile show_early_output_fn_t show_early_output;
+extern volatile show_early_output_fn_t show_early_output;
 
 extern void init_revisions(struct rev_info *revs, const char *prefix);
 extern int setup_revisions(int argc, const char **argv, struct rev_info *revs, const char *def);