git-gui: _which, only add .exe suffix if not present

The _which function finds executables on $PATH, and adds .exe on Windows
unless -script was given. However, win32.tcl executes "wscript.exe"
and "cscript.exe", both of which fail as _which adds .exe to both. This
is already fixed in git-gui released by Git for Windows. Do so here.

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

git-gui.sh

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