git-gui: use [is_Windows], not bad _shellpath

Commit 7d076d5 (git-gui: handle shell script text filters when
loading for blame, 2011-12-09) added open_cmd_pipe, with special
handling for Windows detected by seeing that _shellpath does not
point to an executable shell. That is bad practice, and is broken by
the next commit that assures _shellpath is valid on all platforms.

Fix this by using [is_Windows] as done for all Windows specific code.

Signed-off-by: Mark Levedahl <[email protected]>
Signed-off-by: Johannes Sixt <[email protected]>

Signed-off-by: Taylor Blau <[email protected]>

Author: mark987

git-gui.sh

@@ -543,7 +543,7 @@ proc is_shellscript {filename} {
 # scripts specifically otherwise just call the filter command.
 proc open_cmd_pipe {cmd path} {
 	global env
-	if {![file executable [shellpath]]} {
+	if {[is_Windows]} {
 		set exe [auto_execok [lindex $cmd 0]]
 		if {[is_shellscript [lindex $exe 0]]} {
 			set run [linsert [auto_execok sh] end -c "$cmd \"\$0\"" $path]