scap backport: Change-Id is case insensitive

• When collecting commit fingerprints, scap backport searches for a "Change-Id" line in the commit messages. This search should be case-insensitive to account for alternate presentations of "Change-Id".
• Throw an error if commit is not found the first time, since it's unlikely to need to re-fetch for the submodule update commit.

Bug: T325074