*.[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>

fmt-merge-msg.h

@@ -2,6 +2,6 @@
 #define FMT_MERGE_MSG_H
 
 extern int merge_log_config;
-extern int fmt_merge_msg_config(const char *key, const char *value, void *cb);
+int fmt_merge_msg_config(const char *key, const char *value, void *cb);
 
 #endif /* FMT_MERGE_MSG_H */