gitk(Windows): avoid inadvertently calling executables in the worktree

Just like CVE-2022-41953 for Git GUI, there exists a vulnerability of
`gitk` where it looks for `taskkill.exe` in the current directory before
searching `PATH`.

Note that the many `exec git` calls are unaffected, due to an obscure
quirk in Tcl's `exec` function. Typically, `git.exe` lives next to
`wish.exe` (i.e. the program that is run to execute `gitk` or Git GUI)
in Git for Windows, and that is the saving grace for `git.exe because
`exec` searches the directory where `wish.exe` lives even before the
current directory, according to
https://www.tcl-lang.org/man/tcl/TclCmd/exec.htm#M24:

	If a directory name was not specified as part of the application
	name, the following directories are automatically searched in
	order when attempting to locate the application:

	    The directory from which the Tcl executable was loaded.

	    The current directory.

	    The Windows 32-bit system directory.

	    The Windows home directory.

	    The directories listed in the path.

The same is not true, however, for `taskkill.exe`: it lives in the
Windows system directory (never mind the 32-bit, Tcl's documentation is
outdated on that point, it really means `C:\Windows\system32`).

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

gitk-git/gitk

@@ -9,6 +9,141 @@ exec wish "$0" -- "$@"
 
 package require Tk
 
+######################################################################
+##
+## Enabling platform-specific code paths
+
+proc is_MacOSX {} {
+	if {[tk windowingsystem] eq {aqua}} {
+		return 1
+	}
+	return 0
+}
+
+proc is_Windows {} {
+	if {$::tcl_platform(platform) eq {windows}} {
+		return 1
+	}
+	return 0
+}
+
+set _iscygwin {}
+proc is_Cygwin {} {
+	global _iscygwin
+	if {$_iscygwin eq {}} {
+		if {[string match "CYGWIN_*" $::tcl_platform(os)]} {
+			set _iscygwin 1
+		} else {
+			set _iscygwin 0
+		}
+	}
+	return $_iscygwin
+}
+
+######################################################################
+##
+## PATH lookup
+
+set _search_path {}
+proc _which {what args} {
+	global env _search_exe _search_path
+
+	if {$_search_path eq {}} {
+		if {[is_Cygwin] && [regexp {^(/|\.:)} $env(PATH)]} {
+			set _search_path [split [exec cygpath \
+				--windows \
+				--path \
+				--absolute \
+				$env(PATH)] {;}]
+			set _search_exe .exe
+		} elseif {[is_Windows]} {
+			set gitguidir [file dirname [info script]]
+			regsub -all ";" $gitguidir "\\;" gitguidir
+			set env(PATH) "$gitguidir;$env(PATH)"
+			set _search_path [split $env(PATH) {;}]
+			# Skip empty `PATH` elements
+			set _search_path [lsearch -all -inline -not -exact \
+				$_search_path ""]
+			set _search_exe .exe
+		} else {
+			set _search_path [split $env(PATH) :]
+			set _search_exe {}
+		}
+	}
+
+	if {[is_Windows] && [lsearch -exact $args -script] >= 0} {
+		set suffix {}
+	} else {
+		set suffix $_search_exe
+	}
+
+	foreach p $_search_path {
+		set p [file join $p $what$suffix]
+		if {[file exists $p]} {
+			return [file normalize $p]
+		}
+	}
+	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 {[file pathtype $cmd] ne "absolute"} {
+			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
+			}
+		}
+	}
+	return $command_line
+}
+
+# Override `exec` to avoid unsafe PATH lookup
+
+rename exec real_exec
+
+proc exec {args} {
+	# skip options
+	for {set i 0} {$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
+}
+
+# Override `open` to avoid unsafe PATH lookup
+
+rename open real_open
+
+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
+}
+
+# End of safe PATH lookup stuff
+
 proc hasworktree {} {
     return [expr {[exec git rev-parse --is-bare-repository] == "false" &&
                   [exec git rev-parse --is-inside-git-dir] == "false"}]