clang-format-diff.py: fix -regex/-iregex matching

While debating the finer points of file extension matching, we somehow missed
the bigger problem that the current code will match anything starting with the
default or user-specified pattern (e.g. lit.site.cfg.in).

Fix this by doing what find(1) does, implicitly wrapping the pattern with ^$.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197608 91177308-0d34-0410-b5e6-96231b3b80d8