Fixes for deprecated APIs

`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>

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

Use __typeof__ GNUC keyword for ISO C compatibility

indexer: Avoid one `mmap(2)`/`munmap(2)` pair per `git_indexer_append` call

Support custom git extensions

Fix coding style for pointer

Fix config parsing for multiline with multiple quoted comment chars

notes: use a buffer internally