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 <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>