Fix #971. Partly resolve #703.

If the hook doesn't have a file extension, then Windows won't know how
to run it and you'll get "[WinError 193] %1 is not a valid Win32
application". It's very likely that it's a shell script of some kind, so
use bash.exe (commonly installed via Windows Subsystem for Linux). We
don't want to run all hooks with bash because they could be .bat files.

Update tests to get several hook ones working. More work necessary to
get commit-msg hook working. The hook writes to the wrong file because
it's not using forward slashes in the path:
C:\Users\idbrii\AppData\Local\Temp\bare_test_commit_msg_hook_successy5fo00du\CUsersidbriiAppDataLocalTempbare_test_commit_msg_hook_successy5fo00duCOMMIT_EDITMSG

The whitespace handling and trailer selection isn't very trivial or good
documented. It therefore seemed easier and less error prone to just call
git to parse the message for the trailers section and remove superfluos
whitespaces.

This reverts commit 53d94b8091b36847bb9e495c76bb5a3ec2a2fdb5.

The reason for the revert is that the commit in question introduced a
regression where certain modules, functions and classes that were
exposed before were no longer exposed.

See https://github.com/gitpython-developers/GitPython/pull/1352#issuecomment-932757204
for additional information.

All `from <module> import *` has now been replaced by
`from <module> import X, Y, ...`.

Contributes to #1349