clang-format: Use git-ls-tree to get file mode in diff mode
clang-format: Use git-ls-tree to get file mode in diff mode

Summary:
If a file has been renamed/deleted from the filesystem and --diff mode
with two commits is active, attempting to get the file's mode will fail.
This change uses git-ls-tree instead to get the correct permissions for
the given revision.

Patch by Luis Hector Chavez!

Reviewers: djasper, lodato

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D26287

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@286212 91177308-0d34-0410-b5e6-96231b3b80d8

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