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>
diff --git a/Makefile b/Makefile
index 20f028f..f697618 100644
--- a/Makefile
+++ b/Makefile
@@ -440,6 +440,7 @@
 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