git-gui: sanitize $PATH on all platforms

git-gui: sanitize $PATH on all platforms

Since 8f23432b38d9 (windows: ignore empty `PATH` elements, 2022-11-23),
git-gui removes empty elements from $PATH, and a prior commit made this
remove all non-absolute elements from $PATH. But, this happens only on
Windows. Unsafe $PATH elements in $PATH are possible on all platforms.
Let's sanitize $PATH on all platforms to have consistent behavior. If a
user really wants the current repository on $PATH, they can add its
absolute name to $PATH.

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