fix #2998: encode mtime as i64 instead of u64

saves files mtime as i64 instead of u64 which enables backup of
files with negative mtime

the catalog_decode_i64 is compatible to encoded u64 values (if < 2^63)
but not reverse, so all "old" catalogs can be read with the new
decoder, but catalogs that contain negative mtimes will decode wrongly
on older clients

also remove the arbitrary maximum value of 2^63 - 1 for
encode_u64 (we just use up to 10 bytes now) and correctly
decode them and update the comments accordingly

adds also test for i64 encode/decode and for compatibility between
u64 encode and i64 decode

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/pxar/catalog.rs

@@ -9,7 +9,7 @@ use std::ffi::CStr;
 pub trait BackupCatalogWriter {
     fn start_directory(&mut self, name: &CStr) -> Result<(), Error>;
     fn end_directory(&mut self) -> Result<(), Error>;
-    fn add_file(&mut self, name: &CStr, size: u64, mtime: u64) -> Result<(), Error>;
+    fn add_file(&mut self, name: &CStr, size: u64, mtime: i64) -> Result<(), Error>;
     fn add_symlink(&mut self, name: &CStr) -> Result<(), Error>;
     fn add_hardlink(&mut self, name: &CStr) -> Result<(), Error>;
     fn add_block_device(&mut self, name: &CStr) -> Result<(), Error>;

src/pxar/create.rs

@@ -535,7 +535,7 @@ impl<'a, 'b> Archiver<'a, 'b> {
 
                 let file_size = stat.st_size as u64;
                 if let Some(ref mut catalog) = self.catalog {
-                    catalog.add_file(c_file_name, file_size, stat.st_mtime as u64)?;
+                    catalog.add_file(c_file_name, file_size, stat.st_mtime)?;
                 }
 
                 let offset: LinkOffset =