buf: common_prefix takes a string array

`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.

Attribute lookups are done on paths relative to the repository.  Fail if
erroneously presented with an absolute path.

Always pass a working-directory relative path to attribute lookups
during checkout.

Resolve absolute paths to be working directory relative when looking up
attributes.  Importantly, now we will _never_ pass an absolute path down
to attribute lookup functions.

When `git_repository_hashfile` is handed an absolute path, it determines
whether the path is within the repository's working directory or not.
This is necessary when there is no `as_path` specified.

If the path is within the working directory, then the given path should
be used for attribute lookups (it is the effective `as_path`).  If it is
not within the working directory, then it is _not_ eligible.

Importantly, now we will _never_ pass an absolute path down to attribute
lookup functions.

Make p_getcwd match the rest of our win32 path handling semantics.

(This is currently only used in tests, which is why this disparity went
unnoticed.)

v1.3.0

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.