🏗 Clean up CircleCI merge SHA logic

[rsimha]

Assorted cleanup after #33165:

• Split off pieces of fetch_merge_commit.sh into compute_merge_commit.sh and get_pr_number.sh (🏗 Persist merge commit for reuse with later CI jobs #33165 (comment))
• Update amphtml-docker-small-executor to use a small docker instance with an updated spec (🏗 Use a small docker instance #33195)
• Properly persist mergeSha so it can be used (and logged) by bundle-size (simply exporting it is insufficient)

[rsimha]

looking for a way to avoid the two-step commit of the file location to curl.

Found a way, but it's probably too much trouble to do this dynamically in yaml, not to mention the chicken-and-egg problem of determining the ref. Anyhow, here it is (see URL params):

# Download PR version of a file
curl -H 'Accept: application/vnd.github.v3.raw' \
https://api.github.com/repos/ampproject/amphtml/contents/.circleci/fetch_merge_commit.sh?ref=refs/pull/33215/merge

# Download master version of a file
curl -H 'Accept: application/vnd.github.v3.raw' \
https://api.github.com/repos/ampproject/amphtml/contents/.circleci/fetch_merge_commit.sh?ref=master

[rsimha]

@jridgewell Here's an updated / cleaned up config. Once you review / approve and we have a green build, I'm inclined to modify the curl locations to master and force-merge to avoid the two-step checkin. WDYT?

[rsimha]

At long last, we have everything working end-to-end. For posterity, here's verification:

• bundle-size: Use mergeSha when showing code changes amp-github-apps#1241 was merged and deployed, so the bundle-size bot now accepts a mergeSha field.
• The fully green CircleCI workflow is here
• The binaries were built when 511cc8c was the tip of master (see logs)
• Before the bundle-size stage could begin, e0c3c12 was merged to master, moving the tip forward
• The bundle size job still (correctly) uses 511cc8c as the baseline (even though it is no longer tip of master) (see logs)
• The check run shows zero bundle-size changes and the correct set of code diffs (based on a mergeSha of c088f03)

Based on this, I'm going to push a new commit that updates the URLs from rsimha/amphtml/2021-03-11-CircleConfigUpdates to ampproject/amphtml/master (736dd0a) and merge this PR.