windows: fix Repository>Explore Working Copy

Since Git v2.39.1, we are a bit more stringent in searching the PATH. In
particular, we specifically require the `.exe` suffix.

However, the `Repository>Explore Working Copy` command asks for
`explorer.exe` to be found on the `PATH`, which _already_ has that
suffix.

Let's unstartle the PATH-finding logic about this scenario.

This fixes git-for-windows#4356

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

git-gui/git-gui.sh

@@ -101,6 +101,9 @@ proc _which {what args} {
 
 	if {[is_Windows] && [lsearch -exact $args -script] >= 0} {
 		set suffix {}
+	} elseif {[string match *$_search_exe $what]} {
+		# The search string already has the file extension
+		set suffix {}
 	} else {
 		set suffix $_search_exe
 	}