mingw: hide '.git' dir

The '.git' folder of a non-bare git-repo is special; it exists
for git's own internal purposes and it's name has been chosen
to make it hidden on Unix-ish systems.

On Windows, a file's hidden-ness is a separate attribute which
must be set explicitly. Do so to get the same behaviour as on
Unix.

Test by Pat Thoyts.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
mingw: support automatically hiding dot-files

For cross-platform projects, it can be useful to hide files and
directories starting with a dot, as this is the behavior on
Unix/MacOSX.

However, at least Eclipse has problems interpreting the hidden
flag correctly, so the default is not to hide anything.

Contains improvements by Johannes Schindelin.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
git for windows: fallback and warn for dotgitonly

Git for Windows used to support setting core.hideDotFiles to
dotGitOnly. Hiding .git and other dotfiles are now separate
config options. Tell the user to update his or her config,
and emulate the old behaviour.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>

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This patch is probably only appropriate for Git for Windows.
mingw: make .git-hiding configurable

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>

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I dunno. I doubt anyone actually want this. AFAICT, no other
VCS supports configurable hiding of their meta-data.

But if it turns out to be desirable, this can be squashed into
the first commit.