*.[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>

sub-process.h

@@ -42,7 +42,7 @@ struct subprocess_capability {
 /* subprocess functions */
 
 /* Function to test two subprocess hashmap entries for equality. */
-extern int cmd2process_cmp(const void *unused_cmp_data,
+int cmd2process_cmp(const void *unused_cmp_data,
 			   const void *e1,
 			   const void *e2,
 			   const void *unused_keydata);