*.[ch]: remove extern from function declarations using spatch

There has been a push to remove extern from function declarations.
Remove some instances of "extern" for function declarations which are
caught by Coccinelle. Note that Coccinelle has some difficulty with
processing functions with `__attribute__` or varargs so some `extern`
declarations are left behind to be dealt with in a future patch.

This was the Coccinelle patch used:

	@@
	type T;
	identifier f;
	@@
	- extern
	  T f(...);

and it was run with:

	$ git ls-files \*.{c,h} |
		grep -v ^compat/ |
		xargs spatch --sp-file contrib/coccinelle/noextern.cocci --in-place

Files under `compat/` are intentionally excluded as some are directly
copied from external sources and we should avoid churning them as much
as possible.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/tag.h b/tag.h
index e669c3e..e574904 100644
--- a/tag.h
+++ b/tag.h
@@ -11,13 +11,13 @@ struct tag {
 	char *tag;
 	timestamp_t date;
 };
-extern struct tag *lookup_tag(struct repository *r, const struct object_id *oid);
-extern int parse_tag_buffer(struct repository *r, struct tag *item, const void *data, unsigned long size);
-extern int parse_tag(struct tag *item);
-extern void release_tag_memory(struct tag *t);
-extern struct object *deref_tag(struct repository *r, struct object *, const char *, int);
-extern struct object *deref_tag_noverify(struct object *);
-extern int gpg_verify_tag(const struct object_id *oid,
+struct tag *lookup_tag(struct repository *r, const struct object_id *oid);
+int parse_tag_buffer(struct repository *r, struct tag *item, const void *data, unsigned long size);
+int parse_tag(struct tag *item);
+void release_tag_memory(struct tag *t);
+struct object *deref_tag(struct repository *r, struct object *, const char *, int);
+struct object *deref_tag_noverify(struct object *);
+int gpg_verify_tag(const struct object_id *oid,
 		const char *name_to_report, unsigned flags);
 
 #endif /* TAG_H */