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

sha256: less hardcoded SHA1 types and lengths

xdiff: move xdiff to 'deps'

meta: the main branch is now v1.7.0

config: return `GIT_ENOTFOUND` for missing programdata

When the programdata path is missing, ensure that we pass down the
`GIT_ENOTFOUND` error to the caller instead of converting it to a
generic `-1`.

The config file checksum is only used to detect changes internally -- it
is not stored within the repository. Therefore this need not be
configurable.

Use SHA256 everywhere, regardless of the repository object format.

Our thread-local sha string buffer should be sized to support SHA256
strings.