Add check for _T() to svn pre-commit hook.

Add check for _T() to svn pre-commit hook.

_T() shouldn't be used in any new code so don't allow committing it.

We could have used "svnlook diff" instead of "svnlook cat" and check only for
_T() not being added but in practice there shouldn't be any difference and
using "cat" is simpler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70307 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775