lib/checkout: add filter API to skip over files

This is analogous to the filtering support for the commit API: we allow
library users to skip over checking out specific files. This is useful
in some tricky situations where we *know* that the files to be checked
out will conflict with existing files in subtle ways.

One such example is in rpm-ostree support for multilib. There, we want
to allow checking out a package onto an existing tree, but skipping over
files that are not coloured to our preferred value (e.g. not overwriting
an i686 version of `ldconfig` if we already have the `x86_64` version).
See https://github.com/projectatomic/rpm-ostree/pull/1227 for details.

Closes: #1441
Approved by: cgwalters

src/libostree/ostree-core-private.h

@@ -84,6 +84,7 @@ _ostree_make_temporary_symlink_at (int             tmp_dirfd,
                                    GError        **error);
 
 GFileInfo * _ostree_stbuf_to_gfileinfo (const struct stat *stbuf);
+void _ostree_gfileinfo_to_stbuf (GFileInfo    *file_info, struct stat  *out_stbuf);
 gboolean _ostree_gfileinfo_equal (GFileInfo *a, GFileInfo *b);
 gboolean _ostree_stbuf_equal (struct stat *stbuf_a, struct stat *stbuf_b);
 GFileInfo * _ostree_mode_uidgid_to_gfileinfo (mode_t mode, uid_t uid, gid_t gid);