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.

The diff hunk context string that is returned to xdiff need not
be NUL terminated because the xdiff code just copies the number of
bytes that you report directly into the output.  There was an off
by one in the diff driver code when the header context was longer
than the output buffer size, the output buffer length included
the NUL byte which was copied into the hunk header.

Fixes #1710

Segmentation fault on git_clone

Fixes #1700.

This option serves no benefit now that the git_status_list API
is available.  It was of questionable value before and now it
would just be a bad idea to use it rather than the indexed API.