git-gui: Always disable the Tcl EOF character when reading

git-gui: Always disable the Tcl EOF character when reading

On Windows (which includes Cygwin) Tcl defaults to leaving the EOF
character of input file streams set to the ASCII EOF character, but
if that character were to appear in the data stream then Tcl will
close the channel early.  So we have to disable eofchar on Windows.
Since the default is disabled on all platforms except Windows, we
can just disable it everywhere to prevent any sort of read problem.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>