When a worktree is not prunable, an error message will be set with
information about why. Document that.

Make socket I/O non-blocking and add optional timeouts.

Users may now set `GIT_OPT_SET_SERVER_CONNECT_TIMEOUT` to set a shorter
connection timeout. (The connect timeout cannot be longer than the
operating system default.) Users may also now configure the socket read
and write timeouts with `GIT_OPT_SET_SERVER_TIMEOUT`.

By default, connects still timeout based on the operating system
defaults (typically 75 seconds) and socket read and writes block.

Add a test against our custom testing git server that ensures that we
can timeout reads against a slow server.

git_oid_tostr_s could fail if thread-local state initialization fails.
In that case, it will now return `NULL`.  Callers should check for
`NULL` and propagate the failure.

Shallow (#6396) with some fixes from review

The `depth` field is suitable to specify unshallowing; provide an enum
to aide in specifying the `unshallow` value.

Users should provide us an array of object ids; we don't need a separate
type. And especially, we should not be mutating user-providing values.
Instead, use `git_oid *` in the shallow code.

Depth of `0` should indicate full depth. Disallow negative values (they
may have a future meaning) and use `0` as the default.

The opt mechanism isn't _really_ meant to be for feature flags, and it's
weird to feature flag shallow / unshallow at all.