pkg/vcs: search cherry-picks by title

We sometimes cherry-pick fixes to a bisected branch, for issues that
make large parts of history untestable. Previously we cherry-picked
if the fix commit hash isn't already present.

This is incorrect, as forks / lts trees may already cherry-picked the
fix. In this case the fix would be present, but not have the expected
hash. Unfortunately git doesn't have Change-Ids like gerrit, so there
is no great way to check if a fix is already present.

Instead we now just check whether any commit with the expected title
is present.

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