Bump version to 8.1.0396

- 8.1.0389: :behave command is not tested. Add a test. (Dominique Pelle,
  closes #3429)
- 8.1.0390: Scrollbars are not tested. Add test_scrollbar() and a test.
- 8.1.0391: Building in a shadow directory fails. Don't link the xdiff
  directory but what's in it. (closes #3428)
- 8.1.0392: Error while typing :/foo/s// with 'incsearch' enabled. Do
  not give search errors when highlighting matches.
- 8.1.0393: Not all white space difference options available. Add
  "iblank", "iwhiteall" and "iwhiteeol" to 'diffopt'.
- 8.1.0394: Diffs are not always updated correctly. When using internal
  diff update for any changes properly.
- 8.1.0395: Compiler warning on 64-bit MS-Windows. Add type cast. (Mike
  Williams)
- 8.1.0396: Another compiler warning on 64-bit MS-Windows. Add type
  cast. (Mike Williams)