clang-format: Add an option to git-clang-format to diff between to commits
clang-format: Add an option to git-clang-format to diff between to commits

Summary:
When building pre-upload hooks using git-clang-format, it is useful to limit the scope to a diff of two commits (instead of from a commit against the working tree) to allow for less false positives in dependent commits.

This change adds the option of specifying two git commits to git-clang-format when using the `--diff` flag, which uses a different strategy to diff (using `git-diff-tree` instead of `git-diff-index`), and runs clang-format against the second commit instead of the working directory.

There is a slight backwards-incompatibility introduced with this change: if a filename matches a branch name or other commit-ish, then `git clang-format <commit> <file>` will no longer work as expected; use `git clang-format <commit> -- <file>` instead.

Patch by Luis Hector Chavez!

Reviewers: djasper, lodato

Subscribers: lodato, cfe-commits, srhines

Projects: #clang-c

Differential Revision: https://reviews.llvm.org/D24319

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@282136 91177308-0d34-0410-b5e6-96231b3b80d8

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