sparse: Fix an "symbol 'merge_file' not decared" warning

In order to fix the warning, we add a new "merge-file.h" header
containing the extern declaration of the merge_file() function,
and include the header in the source files that require the
declaration.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -525,6 +525,7 @@ LIB_H += list-objects.h
 LIB_H += ll-merge.h
 LIB_H += log-tree.h
 LIB_H += mailmap.h
+LIB_H += merge-file.h
 LIB_H += merge-recursive.h
 LIB_H += notes.h
 LIB_H += notes-cache.h

builtin/merge-tree.c

@@ -3,6 +3,7 @@
 #include "xdiff-interface.h"
 #include "blob.h"
 #include "exec_cmd.h"
+#include "merge-file.h"
 
 static const char merge_tree_usage[] = "git merge-tree <base-tree> <branch1> <branch2>";
 static int resolve_directories = 1;
@@ -54,8 +55,6 @@ static const char *explanation(struct merge_list *entry)
 	return "removed in remote";
 }
 
-extern void *merge_file(const char *, struct blob *, struct blob *, struct blob *, unsigned long *);
-
 static void *result(struct merge_list *entry, unsigned long *size)
 {
 	enum object_type type;

merge-file.c

@@ -3,6 +3,7 @@
 #include "xdiff-interface.h"
 #include "ll-merge.h"
 #include "blob.h"
+#include "merge-file.h"
 
 static int fill_mmfile_blob(mmfile_t *f, struct blob *obj)
 {

merge-file.h

+#ifndef MERGE_FILE_H
+#define MERGE_FILE_H
+
+extern void *merge_file(const char *path, struct blob *base, struct blob *our,
+			struct blob *their, unsigned long *size);
+
+#endif