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: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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)
 {