git-gui: Make TclTk 8.6 the minimum, allow 8.7

git-gui: Make TclTk 8.6 the minimum, allow 8.7

git-gui requires that Tcl and Tk are 8.5, though the check using
'package require' allows 8.6. As git-gui runs under wish, both Tcl and
Tk are always available and of the same version, so only one need be
checked.

The 8.5 requirement is very outdated as the earliest Tcl currently
shipping on any supported OS is 8.6. 8.7 is in alpha test and is
generally compatible with 8.6, so should also be allowed.  Tcl 9.0 has
planned compatibility breaking changes so cannot be allowed.

Let's update the requirements to be 8.6 or 8.7, and check only on Tcl as
Tk will be the same version.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>