Add --no-ext-diff option to git diff
Add --no-ext-diff option to git diff

When a developer has explicitly set an external diff program to use with
`git`, the `extract_modified_lines` method would hang.

Add the `--no-ext-diff` flag to force git to use the built-in diff
program.

Change-Id: Icd9c4d7af7e0ac3292173468dc80bc5abfaef0d2
Reviewed-on: https://gerrit.causes.com/29550
Tested-by: jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@causes.com>
Reviewed-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>

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