build maint branches on appveyor

Implementation of url.*.insteadOf

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

Rename GIT_EMERGECONFLICT to GIT_ECONFLICT

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.

Changed README to use new OCaml bindings to git

Include conflicts when diffing

Fill the pointers for matching refspecs

Tackle remote API issues from bindings

If there exists a conflict in the index, but no file in the working
directory, this implies that the user wants to accept the resolution
by removing the file.  Thus, remove the conflict entry from the
index, instead of trying to add a (nonexistent) file.

Mark the `old_file` and `new_file` sides of a delta with a new bit,
`GIT_DIFF_FLAG_EXISTS`, that introduces that a particular side of
the delta exists in the diff.

This is useful for indicating whether a working directory item exists
or not, in the presence of a conflict.  Diff users may have previously
used DELETED to determine this information.

When confronted with a conflict in the index, `git_index_add_all`
should stage the working directory copy.  If there is no file in the
working directory, the conflict should simply be removed.

cl_git_sandbox_init_new() will create a clar temp directory and
initialize a new repository at that location.

It's not always obvious the mapping between stage level and
conflict-ness.  More importantly, this can lead otherwise sane
people to write constructs like `if (!git_index_entry_stage(entry))`,
which (while technically correct) is unreadable.

Provide a nice method to help avoid such messy thinking.

Since a diff entry only concerns a single entry, zero the information
for the index side of a conflict.  (The index entry would otherwise
erroneously include the lowest-stage index entry - generally the
ancestor of a conflict.)

Test that during status, the index side of the conflict is empty.

Make sure that we provide a blanked nitem side when the item does not
exist in the working directory.

When diffing against an index, return a new `GIT_DELTA_CONFLICTED`
delta type for items that are conflicted.  For a single file path,
only one delta will be produced (despite the fact that there are
multiple entries in the index).

Index iterators now have the (optional) ability to return conflicts
in the index.  Prior to this change, they would be omitted, and callers
(like diff) would omit conflicted index entries entirely.

If `git_index_remove_bypath` does no work, and returns an OK error
code, it should not set an error message.

When adding a conflict for some path, remove the staged entry.
Otherwise, an illegal index (with both stage 0 and high-stage
entries) would result.

Wrap the iterator current / advance functions so that we can extend
them, but also handle GIT_ITEROVER cases in the iterator funcs
instead of the callers.

These are updates, same as the rest, we should call this callback. As we
are using the callback, let's make sure to skip unnecessary updates.

When we moved from acting on the instance to acting on the
configuration, we dropped the validation of the passed refspec, which
can lead to writing an invalid refspec to the configuration. Bring that
validation back.

An anonymous remote is not configured and cannot therefore have
configured refspecs. Remove the parameter which adds this from the
constructor.

Fix ident replacement to match Git behavior

The public key field is optional and as such can take NULL. Account for
that and do not call strlen() on NULL values. Also assert() for non-NULL
values of username & private key.

Declare GIT_CREDTYPE_SSH_MEMORY to have consistent API independently of
whether libgit2 was built with or without in-memory key passing support.
Or rather, to have it at all since build-time definitions are not stored
in headers.

To not modify the external api.