diff: update `GIT_DIFF_IGNORE_BLANK_LINES`

`GIT_DIFF_IGNORE_BLANK_LINES` needs to be within a (signed) int, per the
`enum` definition of ISO C.

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

ci: pull libssh2 from www.libssh2.org

libssh2.org and www.libssh2.org were previously identical; now this is a
redirect.

Fixes for deprecated APIs

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`

win32: allow empty environment variables

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.

Stdint header condition has been reverted.

The configuration change will override the environment variable, so the
environment variable check needs to come first.

Allow proxy options when connecting with a detached remote.

merge: Check file mode when resolving renames

Initialise cfg variable.

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

Update README.md

`git_diff_format_email` is deprecated in favor of `git_email_create`.

`git format-patch` includes diffs with rename detection enabled by
default when creating emails.  Match this behavior.

`git format-patch` includes binary diffs by default when creating
emails.  Match this behavior.

Introduce `git_email__append_from_diff` so that we don't always
overwrite the input buffer.

Move the `git_diff_commit_as_email` function to use `email_create`.

Allow a `0` patch index and `0` patch count; in this case, simply don't
display these in the email.

Introduce a function to create an email from a diff and multiple inputs
about the source of the diff.

Creating an email from a diff requires many more inputs, and should be
discouraged in favor of building directly from a commit, and is thus in
the `sys` namespace.

Create `git_email_*` which will encapsulate email creation and
application, and `git_email_create_from_commit` in particular, which
creates an email for a single commit.

When determining if ours or theirs changed, we check the oids but not
their respective file modes. This can lead to merges introducing incorrect
file mode changes (eg., in a revert). A simple linear example might be:

commit A - introduces file `foo` with chmod 0755
commit B - updates some unrelated file
commit C - renames `foo` to `bar` and chmod 0644

If B is reverted, `bar` will unexpectedly acquire mode 0755.

Discover libssh2 without pkg-config

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