git-gui: override exec and open only on Windows

Since aae9560 (Work around Tcl's default `PATH` lookup,
2022-11-23), git-gui overrides exec and open on all platforms. But,
this was done in response to Tcl adding elements to $PATH on Windows,
while exec, open, and auto_execok honor $PATH as given on all other
platforms.

Let's do the override only on Windows, restoring others to using their
native exec and open. These honor the sanitized $PATH as that is written
out to env(PATH) in a previous commit. auto_execok is also safe on these
platforms, so can be used for _which.

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

Signed-off-by: Taylor Blau <me@ttaylorr.com>

Author: mark987

git-gui.sh

@@ -112,81 +112,91 @@ unset _path_seen
 
 set env(PATH) [join $_search_path $_path_sep]
 
-proc _which {what args} {
-	global _search_exe _search_path
-
-	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
-	}
+if {[is_Windows]} {
+	proc _which {what args} {
+		global _search_exe _search_path
+
+		if {[lsearch -exact $args -script] >= 0} {
+			set suffix {}
+		} elseif {[string match *$_search_exe [string tolower $what]]} {
+			# The search string already has the file extension
+			set suffix {}
+		} else {
+			set suffix $_search_exe
+		}
 
-	foreach p $_search_path {
-		set p [file join $p $what$suffix]
-		if {[file exists $p]} {
-			return [file normalize $p]
+		foreach p $_search_path {
+			set p [file join $p $what$suffix]
+			if {[file exists $p]} {
+				return [file normalize $p]
+			}
 		}
+		return {}
 	}
-	return {}
-}
 
-proc sanitize_command_line {command_line from_index} {
-	set i $from_index
-	while {$i < [llength $command_line]} {
-		set cmd [lindex $command_line $i]
-		if {[llength [file split $cmd]] < 2} {
-			set fullpath [_which $cmd]
-			if {$fullpath eq ""} {
-				throw {NOT-FOUND} "$cmd not found in PATH"
+	proc sanitize_command_line {command_line from_index} {
+		set i $from_index
+		while {$i < [llength $command_line]} {
+			set cmd [lindex $command_line $i]
+			if {[llength [file split $cmd]] < 2} {
+				set fullpath [_which $cmd]
+				if {$fullpath eq ""} {
+					throw {NOT-FOUND} "$cmd not found in PATH"
+				}
+				lset command_line $i $fullpath
+			}
+
+			# handle piped commands, e.g. `exec A | B`
+			for {incr i} {$i < [llength $command_line]} {incr i} {
+				if {[lindex $command_line $i] eq "|"} {
+					incr i
+					break
+				}
 			}
-			lset command_line $i $fullpath
 		}
+		return $command_line
+	}
+
+	# Override `exec` to avoid unsafe PATH lookup
+
+	rename exec real_exec
 
-		# handle piped commands, e.g. `exec A | B`
-		for {incr i} {$i < [llength $command_line]} {incr i} {
-			if {[lindex $command_line $i] eq "|"} {
+	proc exec {args} {
+		# skip options
+		for {set i 0} {$i < [llength $args]} {incr i} {
+			set arg [lindex $args $i]
+			if {$arg eq "--"} {
 				incr i
 				break
 			}
+			if {[string range $arg 0 0] ne "-"} {
+				break
+			}
 		}
+		set args [sanitize_command_line $args $i]
+		uplevel 1 real_exec $args
 	}
-	return $command_line
-}
 
-# Override `exec` to avoid unsafe PATH lookup
+	# Override `open` to avoid unsafe PATH lookup
 
-rename exec real_exec
+	rename open real_open
 
-proc exec {args} {
-	# skip options
-	for {set i 0} {$i < [llength $args]} {incr i} {
-		set arg [lindex $args $i]
-		if {$arg eq "--"} {
-			incr i
-			break
-		}
-		if {[string range $arg 0 0] ne "-"} {
-			break
+	proc open {args} {
+		set arg0 [lindex $args 0]
+		if {[string range $arg0 0 0] eq "|"} {
+			set command_line [string trim [string range $arg0 1 end]]
+			lset args 0 "| [sanitize_command_line $command_line 0]"
 		}
+		uplevel 1 real_open $args
 	}
-	set args [sanitize_command_line $args $i]
-	uplevel 1 real_exec $args
-}
-
-# Override `open` to avoid unsafe PATH lookup
 
-rename open real_open
+} else {
+	# On non-Windows platforms, auto_execok, exec, and open are safe, and will
+	# use the sanitized search path. But, we need _which for these.
 
-proc open {args} {
-	set arg0 [lindex $args 0]
-	if {[string range $arg0 0 0] eq "|"} {
-		set command_line [string trim [string range $arg0 1 end]]
-		lset args 0 "| [sanitize_command_line $command_line 0]"
+	proc _which {what args} {
+		return [lindex [auto_execok $what] 0]
 	}
-	uplevel 1 real_open $args
 }
 
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