This adds an additional pathspec API that will match a pathspec
against a diff object.  This is convenient if you want to handle
renames (so you need the whole diff and can't use the pathspec
constraint built into the diff API) but still want to tell if the
diff had any files that matched the pathspec.

When the pathspec is matched against a diff, instead of keeping
a list of filenames that matched, instead the API keeps the list
of git_diff_deltas that matched and they can be retrieved via a
new API git_pathspec_match_list_diff_entry.

There are a couple of other minor API extensions here that were
mostly for the sake of convenience and to reduce dependencies
on knowing the internal data structure between files inside the
library.

This is a simple bit vector object that is not resizable after
the initial allocation but can be of arbitrary size.  It will
keep the bti vector entirely on the stack for vectors 64 bits
or less, and will allocate the vector on the heap for larger
sizes.  The API is uniform regardless of storage location.

This is very basic right now and all the APIs are inline functions,
but it is useful for storing an array of boolean values.

This converts the array of parent SHAs from a git_vector where
each SHA has to be separately allocated to a git_array_t where
all the SHAs can be kept in one block.  Since the two collections
have almost identical APIs, there isn't much involved in making
the change.  I did add an API to git_array_t so that it could be
allocated at a precise initial size.

This fixes the way the example log program decides if a merge
commit should be shown when a pathspec is given.  Also makes it
easier to use the pathspec API to just check "does a tree match
anything in the pathspec" without allocating a match list.

This adds more command line processing to the example version of
log.  In particular, this adds the funky command line processing
that allows an arbitrary series of revisions followed by an
arbitrary number of paths and/or glob patterns.

The actual logging part still isn't implemented.

This adds a new public API for compiling pathspecs and matching
them against the working directory, the index, or a tree from the
repository.  This also reworks the pathspec internals to allow the
sharing of code between the existing internal usage of pathspec
matching and the new external API.

While this is working and the new API is ready for discussion, I
think there is still an incorrect behavior in which patterns are
always matched against the full path of an entry without taking
the subdirectories into account (so "s*" will match "subdir/file"
even though it wouldn't with core Git).  Further enhancements are
coming, but this was a good place to take a functional snapshot.

Try harder not to double free index entries

PR 1706 plus error reporting cleanups

The SSH error checking and reporting could still be further
improved by using the libssh2 native methods to get error info,
but at least this ensures that all error codes are checked and
translated into libgit2 error messages.

Conflicts:
	src/transports/ssh.c

Remove GIT_STATUS_SHOW_INDEX_THEN_WORKDIR option

API should not be ifdeffed

Fix git_config_get_multivar with interleaved missing entries

The old tests didn't try failing lookups or lookups across
multiple config files with some having the pattern and some
not having it.

If there is not an error, the return value was always the return value
of the last call to file->get_multivar

With this commit GIT_ENOTFOUND is only returned if all the calls to
filge-get_multivar return GIT_ENOTFOUND.

This also adds a test that actually calls git_libgit2_capabilities
and git_libgit2_version.

This makes all of the credential objects use the same pattern to
clear the contents and call git__memzero when done.  Much of this
information is probably not sensitive, but it also seems better
to just clear consistently.

Null byte in hunk header

Much of the SSH credential creation API can be left enabled even
on platforms with no SSH support.  We really just have to give an
error when you attempt to open the SSH connection.

The SSH APIs will just return an error code and state that the
library was built without SSH support if they are called in
that case.

Make the git_signature `const` in the stash API.