repo: Move commit code to another file

ostree-repo.c is a bit too big, and most of the commit code is
fairly standalone.

https://bugzilla.gnome.org/show_bug.cgi?id=707644

Makefile-libostree.am

@@ -34,6 +34,7 @@ libostree_1_la_SOURCES = \
 	src/libostree/ostree-mutable-tree.c \
 	src/libostree/ostree-repo.c \
 	src/libostree/ostree-repo-checkout.c \
+	src/libostree/ostree-repo-commit.c \
 	src/libostree/ostree-repo-libarchive.c \
 	src/libostree/ostree-repo-prune.c \
 	src/libostree/ostree-repo-refs.c \

src/libostree/ostree-repo-private.h

@@ -70,6 +70,20 @@ GFile *
 _ostree_repo_get_file_object_path (OstreeRepo   *self,
                                    const char   *checksum);
 
+gboolean
+_ostree_repo_find_object (OstreeRepo           *self,
+                          OstreeObjectType      objtype,
+                          const char           *checksum,
+                          GFile               **out_stored_path,
+                          GCancellable         *cancellable,
+                          GError             **error);
+
+gboolean
+_ostree_repo_get_loose_object_dirs (OstreeRepo       *self,
+                                    GPtrArray       **out_object_dirs,
+                                    GCancellable     *cancellable,
+                                    GError          **error);
+
 GFile *
 _ostree_repo_get_object_path (OstreeRepo   *self,
                               const char   *checksum,