Add `const` qualifier

This allows the user to look up fields which we don't parse in libgit2,
and allows them to access gpgsig or mergetag fields if they wish to
check the signature.

We currently use a timetamp to check whether an index file has been
modified since we last read it, but this is racy. If two updates happen
in the same second and we read after the first one, we won't detect the
second one.

Instead read the SHA-1 checksum of the file, which are its last 20 bytes which
gives us a sure-fire way to detect whether the file has changed since we
last read it.

As we're now keeping track of it, expose an accessor to this data.

Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.

cred_helpers: Add 'const' qualifiers to git_cred_userpass_payload

`git_filter_list_contains` can be used to query a filter list to
determine if a given filter will be run.

A remote's URLs are now modified according to the url.*.insteadOf
and url.*.pushInsteadOf configurations. This allows a user to
replace URL prefixes by setting the corresponding keys. E.g.
"url.foo.insteadOf = bar" would replace the prefix "bar" with the
new prefix "foo".

Support getting SSH keys from memory, pt. 2

We do not error on "merge conflicts"; on the contrary, merge conflicts
are a normal part of merging.  We only error on "checkout conflicts",
where a change exists in the index or the working directory that would
otherwise be overwritten by performing the checkout.

This *may* happen during merge (after the production of the new index
that we're going to checkout) but it could happen during any checkout.

Include conflicts when diffing