Support for explicit Git object formats in .buckconfig

[mmhat]

This PR adds a new optional config option for external cells retrieved via Git:

[external_cell_foo]
git_origin = ...
commit_hash = ...
object_format = sha1

[external_cell_bar]
git_origin = ...
commit_hash = ...
object_format = sha256

If specified, we pass --object-format sha1 or --object-format sha256 as arguments to the git init invocation.
In both cases we validate that the commit_hash is a valid digest for the respective algorithm. If no object_format is given, we check that the commit_hash is a valid SHA1 digest, but we do not pass the --object-format option when initializing the Git repository.

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[meta-codesync]

@Nero5023 has imported this pull request. If you are a Meta employee, you can view this in D83523127. (Because this pull request was imported automatically, there will not be any future comments.)

[Nero5023]

Sorry for late reply. Thank you for doing it!
Could you do a rebase?

[mmhat]

Sorry for late reply. Thank you for doing it! Could you do a rebase?

@Nero5023 No problem; I rebased the commit 👍