git-gui: avoid auto_execok for git-bash menu item

On Windows, git-gui offers to open a git-bash session for the current
repository from the menu, but uses [auto_execok start] to get the
command to actually run that shell.

The code for auto_execok, in /usr/share/tcl8.6/tcl.init, has 'start' in
the 'shellBuiltins' list for cmd.exe on Windows: as a result,
auto_execok does not actually search for start, meaning this usage is
technically ok with auto_execok now.  However, leaving this use of
auto_execok in place will just induce confusion about why a known unsafe
function is being used on Windows. Instead, let's switch to using our
known safe _which function that looks only in $PATH, excluding the
current working directory.

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

@@ -2769,7 +2769,7 @@ if {[is_Windows]} {
 	}
 	.mbar.repository add command \
 		-label [mc "Git Bash"] \
-		-command {eval exec [auto_execok start] $cmdLine}
+		-command {eval exec [list [_which cmd] /c start] $cmdLine}
 }
 
 if {[is_Windows] || ![is_bare]} {