odb: A couple internal improvements to body parsing/rendering

[rwe]

A few miscellaneous fixes related to Commit.body and Tree.body

• Fix: space-indented commit header value continuation lines were handled on parsing, but not restored on construction.
• Fix: Commit header key/value parsing just used split(maxsplit=1), but that will incorrectly lose leading whitespace from the value. Instead, it now explicitly runs .split(b" ", maxsplit=1) to designate exactly one space character as the separator. This is consistent with how git internally parses those values.
• Style/performance: Previously, bodies were constructed by repeatedly appending to an immutable bytestring. This has O(n^2) memory usage due to copying each time. (Some python runtimes apparently detect this if the variable isn't used for anything else, but as far as I can find it's still frowned upon). Now, the body parts are yielded from a generator, which is given to b"".join(…) to construct that contiguous bytestring all at once, giving the internals more leeway for buffer handling.

[rwe]

I've rebased this to cover the gpgsig changes along with a couple tiny related cleanups in that area.

[rwe]

This branch has been good to go for a few months but I've pushed up a rebase to fix a mypy checking issue introduced by a conflict with the recently introduced sh_run utility function.