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>