Fix new git based diff generation file paths (#147)

* Fix new git based diff generation file paths

* Update cmd/src/actions_exec_backend_runner.go

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

cmd/src/actions_exec_backend_runner.go

@@ -239,7 +239,11 @@ func runAction(ctx context.Context, prefix, repoID, repoName, rev string, steps
 		return nil, errors.Wrap(err, "git add failed")
 	}
 
-	diffOut, err := runGitCmd("diff", "--cached")
+	// As of Sourcegraph 3.14 we only support unified diff format.
+	// That means we need to strip away the `a/` and `/b` prefixes with `--no-prefix`.
+	// See: https://github.com/sourcegraph/sourcegraph/blob/82d5e7e1562fef6be5c0b17f18631040fd330835/enterprise/internal/campaigns/service.go#L324-L329
+	// 
+	diffOut, err := runGitCmd("diff", "--cached", "--no-prefix")
 	if err != nil {
 		return nil, errors.Wrap(err, "git diff failed")
 	}