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.

`git_filebuf` hashes contents as its written; allow for SHA1 or SHA256
as that hash. Currently, most callers simply use SHA1 as they do not yet
know about SHA256 themselves.

repo: honor environment variables for more scenarios

When the repository is opened with `GIT_REPOSITORY_OPEN_FROM_ENV`, honor
the `GIT_CONFIG_GLOBAL`, `GIT_CONFIG_SYSTEM` and `GIT_CONFIG_NOSYSTEM`
environment variables.