[fxtest] Add --affected-since flag for run/show-affected-test

Using run/show-affected-test along with affected-since flag allows
developers to find dirty files that already committed. Also, this flag
accepts arguments just like git command does. For example:
```
fx test --show-affected-test --affected-since HEAD~1
fx test --show-affected-test --affected-since <commit_sha>
```
Default behaviour of run/show-affected-test remains the same, if no
affected-since is provided, it will find the dirty files that haven't
been committed.

Bug: 507487503

Change-Id: I65878fba779e9121f5d2ee5b09685eacc4aeabb5
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1602368
Commit-Queue: michael liao <weijliao@google.com>
Reviewed-by: Christopher Johnson <crjohns@google.com>
Fuchsia-Auto-Submit: michael liao <weijliao@google.com>