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.c

@@ -1686,6 +1686,26 @@ _ostree_stbuf_to_gfileinfo (const struct stat *stbuf)
   return ret;
 }
 
+/**
+ * _ostree_gfileinfo_to_stbuf:
+ * @file_info: File info
+ * @out_stbuf: (out): stat buffer
+ *
+ * Map GFileInfo data from @file_info onto @out_stbuf.
+ */
+void
+_ostree_gfileinfo_to_stbuf (GFileInfo    *file_info,
+                            struct stat  *out_stbuf)
+{
+  struct stat stbuf = {0,};
+  stbuf.st_mode = g_file_info_get_attribute_uint32 (file_info, "unix::mode");
+  stbuf.st_uid = g_file_info_get_attribute_uint32 (file_info, "unix::uid");
+  stbuf.st_gid = g_file_info_get_attribute_uint32 (file_info, "unix::gid");
+  if (S_ISREG (stbuf.st_mode))
+    stbuf.st_size = g_file_info_get_attribute_uint64 (file_info, "standard::size");
+  *out_stbuf = stbuf;
+}
+
 /**
  * _ostree_gfileinfo_equal:
  * @a: First file info