repo: Add a "force copy" flag to checkout

This is intended to be used for copying `/usr/etc` → `/etc` for
deployments.

A TODO here is to use `glnx_file_copy_at()` if the repo mode allows
it - then we'd use reflinks if available.

Closes: #804
Approved by: jlebon

src/libostree/ostree-repo-checkout.c

@@ -403,7 +403,7 @@ checkout_one_file_at (OstreeRepo                        *repo,
 
       need_copy = FALSE;
     }
-  else
+  else if (!options->force_copy)
     {
       HardlinkResult hardlink_res = HARDLINK_RESULT_NOT_SUPPORTED;
       /* Try to do a hardlink first, if it's a regular file.  This also
@@ -895,6 +895,8 @@ ostree_repo_checkout_at (OstreeRepo                        *self,
   if (ostree_repo_get_mode (self) == OSTREE_REPO_MODE_BARE_USER_ONLY)
     options->mode = OSTREE_REPO_CHECKOUT_MODE_USER;
 
+  g_return_val_if_fail (!(options->force_copy && options->no_copy_fallback), FALSE);
+
   g_autoptr(GFile) commit_root = (GFile*) _ostree_repo_file_new_for_commit (self, commit, error);
   if (!commit_root)
     return FALSE;

src/libostree/ostree-repo.h

@@ -767,7 +767,8 @@ typedef struct {
   gboolean enable_fsync;  /* Deprecated */
   gboolean process_whiteouts;
   gboolean no_copy_fallback;
-  gboolean unused_bools[7];
+  gboolean force_copy; /* Since: 2017.6 */
+  gboolean unused_bools[6];
 
   const char *subpath;