ref(vcs): Make extract_pr_{base,head}_sha_from_event match

[chromy]

Make functions extract_pr_{base,head}_sha_from_event match exactly except for
SHA they extract. Previously they have different APIs (Result<Option<String>> vs
Option<String>). We choose Option<String> since there seems little point forcing
users to handle the case of Github returning invalid json. Better options are:

• Returning None
• Crashing (panic!)

We switch both extract_pr_head_sha_from_event and
extract_pr_base_sha_from_event to panic.

[szokeasaurusrex]

lgtm, thank you for explaining the reasoning here.

One reason we might wish to crash instead of just logging a debug message: If GitHub ever changes the schema of the JSON event payload, in a way that this function would fail to extract the SHA, this would fail silently (debug messages are not logged at all by default, as the default log level is WARN). But, I'll leave it up to you to decide which way to go.

[chromy]

One reason we might wish to crash instead of just logging a debug message: If GitHub ever changes the schema of the JSON event payload, in a way that this function would fail to extract the SHA, this would fail silently (debug messages are not logged at all by default, as the default log level is WARN). But, I'll leave it up to you to decide which way to go.

sg! Is it enough to call panic! (I have updated the code to do that) or is there a fancier way to hard error?