`git_strarray` is a public-facing type.  Change
`git_buf_text_common_prefix` to not use it, and just take an array of
strings instead.

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.

Co-authored-by: Edward Thomson <ethomson@github.com>