git-gui: assure PATH has only absolute elements.

git-gui: assure PATH has only absolute elements.

Since 8f23432b38d9 (windows: ignore empty `PATH` elements, 2022-11-23),
git-gui excises all empty paths from $PATH, but still allows '.' or
other relative paths, which can also allow executing code from the
repository. Let's remove anything except absolute elements. While here,
let's remove duplicated elements, which are very common on Windows:
only the first such item can do anything except waste time repeating a
search.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>