git-gui: use -profile tcl8 for file input with Tcl 9

git-gui 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 has long been acceptable on git 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 for representation in unicode.

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 byte by byte 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
git-gui 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.

The tcl8 profile used so far has acceptable behavior given git-gui's
acceptance: this allows git-gui 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.

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

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