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

Git inserts a space after the SHA1 (as of 2.1.4 at least), so do the
same.

Thin packs: don't 

When thickening a pack, avoid loading already loaded bases and
trying to insert them all over again.

When creating an index, know that we do not have an index for
our own packfile, preventing some unnecessary file opens and
error reporting.

openssl: don't try to handle WANT_READ or WANT_WRITE

When we look for which remote corresponds to a remote-tracking branch,
we look in the refspecs to see which ones matches. If none do, we should
abort. We currently ignore the error message from this operation, so
let's not do that anymore.

As part of the test we're writing, let's test for the expected behaviour
if we cannot find a refspec which tells us what the remote-tracking
branch for a remote would look like.

When we find out that we're dealing with a matching refspec, we set the
flag and return immediately. This leaves the strings as NULL, which
breaks the contract.

Assign these pointers to a string with the correct values.

Fix error when building as C++.

index_add_all: include untracked files in new subdirs

No need to get reports about individual ignored files, having a
single ignored directory delta is enough.

ignore: clear the error when matching a pattern negation