core.hidedotfiles: hide '.git' dir by default

At least for cross-platform projects, it makes sense to hide the
files 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 to hide only the .git/ directory.

The config setting core.hideDotFiles therefore supports not only
'true' and 'false', but also 'dotGitOnly'.

[jes: clarified the commit message, made git init respect the setting
by marking the .git/ directory only after reading the config, and added
documentation, and rebased on top of current junio/next]

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/init-db.c

@@ -327,6 +327,7 @@ int init_db(const char *template_dir, unsigned int flags)
 	check_repository_format();
 
 	reinit = create_default_files(template_dir);
+	mark_as_git_dir(get_git_dir());
 
 	create_object_directory();