versions: determine version from a Formula instance
versions: determine version from a Formula instance

Rather than regex the output of `git show <rev>:<path>`, we write the
output of `git cat-file blog <rev>:<path>` to a temporary file, and then
operate on it just as we would a normal formula.

I haven't observed any speed difference. `git cat-file blob` is faster
than `git show`, but the "slow part" is still the git-rev-list
invocation. But really it's pretty fast overall.

Signed-off-by: Jack Nagel <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

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