Previously, only a few characters of the SHA would appear on the preview link
comment posted on the PR. This was usually enough for GitHub to create a link to
the corresponding commit, but it was possible to have collisions with other
commits with the same first characters (which prevented GitHub from identifying
the correct commit and create a link.)
This commit fixes this issue by including the full SHA on the commentso GitHub
can identify the correct commit and create the link. GitHub will automatically
truncate the link text (by default to 7 chars unless more are necessary to
uniquely identify the commit).
Previously, when trying to upload the build artifacts for a PR/SHA that was
already successfully deployed (e.g. when re-running a Travis job), the preview
server would return a 403 and the build would fail.
Since we have other mechanisms to verify that the PR author is trusted and the
artifacts do indeed come from the specified PR and since the new artifacts
should be the same with the already deployed ones (same SHA), there is no reason
to fail the build. The preview server will reject the request with a special
HTTP status code (409 - Conflict), which the `deploy-preview` script will
recognize and exit with 0.
