archive: disable preallocate-contents by default

using fallocate() to preallocate files space does more harm than good:
 - breaks compression on btrfs
 - has been called "not the right thing to do" by xfs developers
(because delayed allocation that most filesystems implement leads to smarter
allocation than what the filesystem needs to do if we upfront fallocate files)

src/libutil/archive.cc

@@ -27,7 +27,7 @@ struct ArchiveSettings : Config
         #endif
         "use-case-hack",
         "Whether to enable a Darwin-specific hack for dealing with file name collisions."};
-    Setting<bool> preallocateContents{this, true, "preallocate-contents",
+    Setting<bool> preallocateContents{this, false, "preallocate-contents",
         "Whether to preallocate files when writing objects with known size."};
 };