patch_sync: Add show subcommand

This allows users to view the combined PATCHES.json,
also known as the source of truth.

Also makes some changes so that serializing the patches
vec to a string is more natural.

BUG=b:209493133
TEST=cargo test
TEST=patch_sync show -s <cros_checkout> <android_checkout>

Change-Id: Ic2054c0c3c7e0bb03b698339b75e9ccc6dd68c74
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/3355361
Reviewed-by: Michael Benfield <mbenfield@google.com>
Tested-by: Jordan Abrahams <ajordanr@google.com>
Commit-Queue: Jordan Abrahams <ajordanr@google.com>
2 files changed