gitk: Fix display of copyright symbol

gitk: Fix display of copyright symbol

The script file uses utf-8 encoding but when sourced it will be read
using the default system encoding which is never utf8 on windows.
This causes the copyright symbol to display incorrectly in the about
dialog.  Using the unicode escape sequence avoids incorrect decoding
but does require a double escape in the .po files.

Also adjusted the year range.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>