Change how commit messages are taken + multiple commits handling
Change how commit messages are taken + multiple commits handling

Multiple commits are separated and format-checked separately.
Commit message is now taken solely from input_api.
Tags paragraph now covers the very last paragraph until the
first line that doesn't have a ":"

Bug: angleproject:4662
Change-Id: I84fe3fd1ffc30f6892a5c9dbe545acf24b0fc595
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2241617
Commit-Queue: Manh Nguyen <7c79ef580b4d963b83ede739cb9e40f930bcb4c9@google.com>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle