Exclude binary files from the checks done by the pre-commit hook.

Exclude binary files from the checks done by the pre-commit hook.

We shouldn't check that binary files are UTF-8 so determine whether the file
is binary by looking at its svn:mime-type and only do the check for the text
files.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62045 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775