setup: prepare setup_discovered_git_dir() for the root directory

setup: prepare setup_discovered_git_dir() for the root directory

Currently, the offset parameter (indicating what part of the cwd
parameter corresponds to the current directory after discovering the
.git/ directory) is set to 0 when we are running in the root directory.

However, in the next patches we will avoid changing the current working
directory while searching for the .git/ directory, meaning that the
offset corresponding to the root directory will have to be 1 to reflect
that this directory is characterized by the path "/" (and not "").

So let's make sure that setup_discovered_git_directory() only tries to
append the trailing slash to non-root directories.

Note: the setup_bare_git_directory() does not need a corresponding
change, as it does not want to return a prefix.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>