clang-format-diff.py: fix -regex/-iregex matching
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@197608 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang