git-gui: Fix gitk search in $PATH to work on Windows

Back in 15430be ("Look for gitk in $PATH, not $LIBEXEC/git-core")
git-gui learned to use [_which gitk] to locate where gitk's script
is as Git 1.6 will install gitk to $prefix/bin (in $PATH) and all
of the other tools are in $gitexecdir.

This failed on Windows because _which adds the ".exe" suffix as it
searches for the program on $PATH, under the assumption that we can
only execute something from Tcl if it is a proper Windows executable.

When scanning for gitk on Windows we need to omit the ".exe" suffix.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -317,7 +317,7 @@ proc _git_cmd {name} {
 	return $v
 }
 
-proc _which {what} {
+proc _which {what args} {
 	global env _search_exe _search_path
 
 	if {$_search_path eq {}} {
@@ -340,8 +340,14 @@ proc _which {what} {
 		}
 	}
 
+	if {[is_Windows] && [lsearch -exact $args -script] >= 0} {
+		set suffix {}
+	} else {
+		set suffix $_search_exe
+	}
+
 	foreach p $_search_path {
-		set p [file join $p $what$_search_exe]
+		set p [file join $p $what$suffix]
 		if {[file exists $p]} {
 			return [file normalize $p]
 		}
@@ -1686,7 +1692,7 @@ proc do_gitk {revs} {
 	# -- Always start gitk through whatever we were loaded with.  This
 	#    lets us bypass using shell process on Windows systems.
 	#
-	set exe [_which gitk]
+	set exe [_which gitk -script]
 	set cmd [list [info nameofexecutable] $exe]
 	if {$exe eq {}} {
 		error_popup [mc "Couldn't find gitk in PATH"]