libarchive: Add support for translating paths during commit

For rpm-ostree, I want to move RPM files in `/boot` to `/usr/lib/ostree-boot`.
This is currently impossible without forking the libarchive code.  Supporting
this is pretty straightforward; we already had pathname translation in
the libarchive code, we just need to expose it as an option.

On the command line side, I chose to wrap this as a regexp. That should be good
enough for a lot of use cases; sophisticated users should as always be making
use of the API. Note that this required some new `#ifdef LIBARCHIVE` bits to use
the new API. Following previous patterns here, we use the new API only if a
relevant option is enabled, ensuring unit test coverage of both paths.

For the test cases, I ended up changing the accounting to avoid having to
multiply the test count.

Closes: #1105
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -22,6 +22,8 @@
 
 #pragma once
 
+#include <sys/stat.h>
+
 #include "ostree-core.h"
 #include "ostree-types.h"
 #include "ostree-async-progress.h"
@@ -688,6 +690,31 @@ gboolean      ostree_repo_write_archive_to_mtree (OstreeRepo                   *
                                                   GCancellable                 *cancellable,
                                                   GError                      **error);
 
+/**
+ * OstreeRepoImportArchiveTranslatePathname:
+ * @repo: Repo
+ * @stbuf: Stat buffer
+ * @src_path: Path in the archive
+ * @user_data: User data
+ *
+ * Possibly change a pathname while importing an archive. If %NULL is returned,
+ * then @src_path will be used unchanged.  Otherwise, return a new pathname which
+ * will be freed via `g_free()`.
+ *
+ * This pathname translation will be performed *before* any processing from an
+ * active `OstreeRepoCommitModifier`. Will be invoked for all directory and file
+ * types, first with outer directories, then their sub-files and directories.
+ *
+ * Note that enabling pathname translation will always override the setting for
+ * `use_ostree_convention`.
+ *
+ * Since: 2017.11
+ */
+typedef char *(*OstreeRepoImportArchiveTranslatePathname) (OstreeRepo     *repo,
+                                                           const struct stat *stbuf,
+                                                           const char     *src_path,
+                                                           gpointer        user_data);
+
 /**
  * OstreeRepoImportArchiveOptions: (skip)
  *
@@ -703,7 +730,9 @@ typedef struct {
   guint reserved : 28;
 
   guint unused_uint[8];
-  gpointer unused_ptrs[8];
+  OstreeRepoImportArchiveTranslatePathname translate_pathname;
+  gpointer translate_pathname_user_data;
+  gpointer unused_ptrs[6];
 } OstreeRepoImportArchiveOptions;
 
 _OSTREE_PUBLIC