pkg/vcs: consider merge commits in base commit detection

Merge commits are important in two contexts:
1) They may bring together blobs created in different commits.
2) They may create new blob hashes when file changes are merged.
3) They replace blob hashes we were looking for.

A particularly unpleasant case for the previous approach is when a blob
hash disappears after a merge and tips of the branches are no longer to
trust as the files of interest have also been replaced by the merges.

Add a test that sets up this complicated setup and change the logic to
make the test pass:
1) Add a -m flag to consider merge commits.
2) Increase -n as there are lots of merge commits in the Linux kernel.
3) Consider all parents of merge commits.

Fixes #6777.

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