Split out merge_recursive() to merge-recursive.c

Move most of the of code from builtin-merge-recursive.c to a new file
merge-recursive.c and introduce merge_recursive_setup() in there so that
builtin-merge-recursive and other builtins call it.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -440,6 +440,7 @@ LIB_OBJS += log-tree.o
 LIB_OBJS += mailmap.o
 LIB_OBJS += match-trees.o
 LIB_OBJS += merge-file.o
+LIB_OBJS += merge-recursive.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += object.o
 LIB_OBJS += pack-check.o

merge-recursive.h

@@ -14,7 +14,11 @@ int merge_trees(struct tree *head,
 		const char *branch1,
 		const char *branch2,
 		struct tree **result);
-
+struct commit *make_virtual_commit(struct tree *tree, const char *comment);
+int merge_recursive_config(const char *var, const char *value, void *cb);
+void merge_recursive_setup(int is_subtree_merge);
 struct tree *write_tree_from_memory(void);
 
+extern int merge_recursive_verbosity;
+
 #endif