Fixed bug where basis file is truncated on remote copy

The truncation of the basis file caused the File Stream API to compute
the signature against an empty file, effectively removing all
performance benefits from the RSYNC algorithm.

Changelog: Title
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

libcfnet/file_stream.c

@@ -1022,8 +1022,8 @@ bool FileStreamFetch(
     assert(basis != NULL);
     assert(dest != NULL);
 
-    /* Let's make sure the basis file exists */
-    FILE *file = safe_fopen_create_perms(basis, "wb", perms);
+    /* Let's make sure the basis file exists, but don't truncate it */
+    FILE *file = safe_fopen_create_perms(basis, "ab+", perms);
     if (file != NULL)
     {
         fclose(file);