🏗 Read merge commit only if file was created

[rsimha]

This PR prevents a race that occurs as follows:

• A file is directly changed on ampproject/amphtml and a branch is created
• CI build starts on that branch
• A PR is created after the first stage of CI has begun, but before the second stage
• Second stage tries to read the merge commit file

Now, we don't read the merge commit file if it doesn't exist.

Example race:

First stage:

Second stage: