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.

When the repository is opened with `GIT_REPOSITORY_OPEN_FROM_ENV`, honor
the `GIT_COMMON_DIR` environment variable.

When the repository is opened with `GIT_REPOSITORY_OPEN_FROM_ENV`, honor
the `GIT_WORK_TREE` environment variable.

Remove the `_git_repository_open_ext_from_env` and make it part of the
general repository opening code path. This removes the re-entrancy, and
standardizes things like index and config opening to a single place
again so that we have predictable ordering of the opening procedure.

"Could not find repository from ..." doesn't make sense. "Could not find
repository _at_ ..." does not indicate that we walked down the path
hierarchy, but at least it's more correct.

Shocked that our leak checkers didn't find this earlier.

repo: don't allow repeated extensions

Introduce Schannel and SSPI for Windows

v1.6.3