core: make fork() work on Windows under Python 3

Use the same approach in _fork_win32() that was used in commit ee990fbc
for start_command().

Signed-off-by: Daniel Harding <dharding@living180.net>
Don't make user acknowledge successful branch creation

When the user creates a new branch, there's no need to show a modal
dialog saying that this succeeded. If the user is interested in any
output from git, they can open the console pane to view it.
Merge pull request #250 from living180/fix_fork_on_windows_py3

Make fork() work on Windows under Python 3
Merge pull request #251 from takluyver/create-branch-noack

Don't make user acknowledge successful branch creation
doc: add Thomas to the credits

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update v2.0.3 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>