The `repo` argument is now unnecessary.  Remove it.

When looking up attributes for a file, we construct an absolute path
to the queried file within the working directory so that we can accept
both absolute paths and working directory relative paths.  We then trim
the leading working directory path to give us an in-repo path.

Since we only want the in-repo path to look up attributes - and not to
read it from disk - we don't need to validate its length.

filter: use a `git_oid` in filter options, not a pointer

Using a `git_oid *` in filter options was a mistake; it is a deviation
from our typical pattern, and callers in some languages that GC may need
very special treatment in order to pass both an options structure and a
pointer outside of it.

`git_email__append_from_diff` is meant to - well, append from a diff.
Clearing the buffer, by definition, is not appending.  Stop doing that.

Introduce `git_email_create`; deprecate `git_diff_format_email`

A length of 0 indicates an error and GetLastError() will be set.  If
GetLastError() is unset then the environment variable has a length of 0.

Allow proxy options when connecting with a detached remote.

Initialise cfg variable.