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

Add support for SSPI on Windows, which offers NTLM and Negotiate
authentication.

Add functions to use convert a string with length, instead of assuming
NUL termination.

In addition, move the utf8 to 16 conversion routines into the `git_utf8`
namespace instead of using namespaceless `git__` prefixed names.

Name the GSSAPI and ntlmclient authentication providers as such. Today
they're named after the authentication mechanism ("Negotiate", "NTLM")
instead of their implementation.

If we have competing implementations for the same mechanism (eg, a
future Windows SSPI-based provider for Negotiate and NTLM) then this
will get confusing.