rhg: in path_encode, be a bit more conservative about memory usage

Use [shrink_to_fit] to match the previous behavior more closely,
and potentially save (a tiny bit) of memory.

FWIW, I suspect this is unnecessary, but this whole MR is
about simplifying things while preserving any existing
optimizations.

rust/hg-core/src/revlog/path_encode.rs

@@ -576,6 +576,7 b' fn hash_mangle(src: &[u8], sha: &[u8]) -'
     if let Some(l) = last_dot {
         dest.write_bytes(&src[l..]);
     }
+    dest.shrink_to_fit();
     dest
 }