gitk: use -profile tcl8 for file input with Tcl 9

gitk: use -profile tcl8 for file input with Tcl 9

gitk invokes many git commands expecting output in utf-8 encoding, but
git accepts extended ascii (code page unknown) as utf-8 without
validating, so cannot guarantee valid utf-8 on output.  In particular,
using any extended ascii code page, of which there are many, has long
been acceptable given that everyone on a project is aware of and uses
that same code page to view all data. utf-8 accepts only 7-bit ascii
characters in single bytes, and any characters outside of that base set
require at least two bytes.

Tcl is a string based language, and transcodes all input data to an
internal unicode format, and to whatever format is requested on output:
"pure" binary is recoded using iso8859-1.  Tcl8.x silently recodes
invalid utf-8 as binary data, so extended ascii characters maintain
their binary value on output but may not display correctly.

Tcl 8.7 added three profiles to control this behaviour: strict (raises
exceptions), replace (replaces each invalid byte with ?), and the
default tcl8 maintaining the old behavior.  Tcl 9 changes the default
profile to strict, meaning any invalid utf-8 raises an exception that
gitk does not handle.

An example of this in the git repository is commit 7eb93c8965 ("[PATCH]
Simplify git script", 2005-09-07). This includes extended ascii
characters in the author name and commit message. As a result, gitk +
Tcl 9 cannot view the git repository at any point beyond that commit.
Note: Tcl 9.0 has a bug, to be fixed in 9.1, where this particular
condition results in a memory error causing Tcl to crash [1].

The tcl8 profile used so far has acceptable behavior given gitk's
acceptance: this allows gitk to accept extended ascii though it may
display incorrectly.  Let's continue that behavior by overriding open to
use the tcl8 profile on Tcl9 and later: Tcl 8.6 does not understand
fconfigure -profile, and Tcl 8.7 maintains the tcl8 profile.

[1] Per https://core.tcl-lang.org/tcl/tktview/73bb42fb3f35cd613af6fcea465e35bbfd352216

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