Function to remove snapshots

[wlandau]

Prework

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Proposal

Suggested by @smwindecker in her rOpenSci review. Plan:

1. Make all git snapshots orphan branches. History will no longer be shared among branches, but there is no loss of efficiency for fully up-to-date files. Efficiency from diffs may be lost, but diffs in target storage may be impossible anyway, I will need to check.
2. Remove commits with no ref: https://stackoverflow.com/questions/3765234/listing-and-deleting-git-commits-that-are-under-no-branch-dangling

[wlandau]

From https://git-scm.com/book/id/v2/Kostumisasi-Git-Git-Attributes, it looks like file diffs rely on filters. Gittargets already uses the lfs filter, which seems to preclude diffs, so orphan branches are probably fine.

[wlandau]

On second thought, orphan branches lose valuable information about the audit trail of which targets changed between code commits. From the perspective of tracking history, I think this is more valuable than the ability to delete data snapshots. Might revisit this issue at some point.

[pvtodorov]

Came across this issue looking for a way to delete entire snapshots. Is there a way to do this yet?

[wlandau]

Unfortunately no, it gets messy because Git tracks differences between one commit and the next. Each successive snapshot depends on the previous one for storage efficiency.