Add support for symlinks to gitiles module

Currently, the gitiles module returns (0, EOF) for symlinks (e.g. default.xml).

BUG=None
TEST=new and existing tests

Change-Id: I119e20d8599fbb7d9e19272cdab106c4128ecc4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2613738
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
3 files changed