Use `git show --format=` rather than `git diff` for post-commit
Use `git show --format=` rather than `git diff` for post-commit

`git diff HEAD~ HEAD` fails if there is no previous commit.

Change-Id: Ib2fcba689575dc599c08df33f1e212e5a111bb65
Reviewed-on: http://gerrit.causes.com/46925
Tested-by: jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@brigade.com>
Reviewed-by: Shane da Silva <67fb7bfe39f841b16a1f86d3928acc568d12c716@brigade.com>

TheSooth/overcommit,TheSooth/overcommit,gnagel/overcommit,oxocode/overcommit,gnagel/overcommit,blech75/overcommit,oxocode/overcommit,blech75/overcommit