Introduce a refs iterator

If the backend doesn't provide support for it, the matching is done in
refdb on top of a normal iterator.

Nobody should ever be using anything other than ALL at this level, so
remove the option altogether.

As part of this, git_reference_foreach_glob is now implemented in the
frontend using an iterator. Backends will later regain the ability of
doing the glob filtering in the backend.

Selecting wether to list loose or packed references is not something
we want to support anymore, so remove a test for this.

This allows us to get a list of reference names in a loop instead of callbacks.

Improve ignore handling in git_status_file

Fix diff output for renames and copies

Fix some memory leaks

If you use rename detection, the renamed and copied files would
not show any text diffs because the function that decides if
data should be loaded didn't know which sides of the diff to
load for those cases.

This adds a test that looks at the patch generated for diff
entries that are COPIED or RENAMED.

The git_status_file API was doing a hack to deal with files that
are inside ignored directories.  The status scan was not reporting
any file in this case, so git_status_file would attempt a final
"stat()" call, and return IGNORED if the file actually existed.

On case-insensitive filesystems where core.ignorecase is set
incorrectly, this magic check can "succeed" and report a file
as ignored when it should actually return ENOTFOUND.

Now that we have the GIT_STATUS_OPT_RECURSE_IGNORED_DIRS, we can
use that flag to make sure that git_status_file() will look into
ignored directories and eliminate the hack completely, so we give
the correct error.

calloc refs instead of malloc'ing them

Fix git_repository_message docs

This clarifies the docs for git_repository_message and also adds
to the tests to explicitly check NUL termination of data when the
output buffer is smaller than the message size.  There is a minor
behavior change so that a non-NULL output buffer will always be
NUL terminated (at length zero) if an error occurs.

Expose git_reference_dwim

Fix windows diff eofnl error

Config helpers for global/xdg config files

Extract this function out of the rev-parse code to be able to DWIM a
reference instead of its target.

The rules for which one to open is a bit silly, so let's make it
easier for our users.

When a repository is initialised, we need to probe to see if there is
a global config to load. If this is not the case, the user isn't able
to write to the global config without creating the backend and adding
it themselves, which is inconvenient and overly complex.

Unconditionally create and add a backend for the global config file
regardless of whether it exists as a convenience for users.

To enable this, we allow creating backends to files that do not exist
yet, changing the semantics somewhat, and making some tests invalid.

remote: correctly interpret tagopt '--tags'

When tagopt is set to '--tags', we should only take the default tags
refspec into account and ignore any configured ones.

Bring the code into compliance.

Diff patch bug fixes

When a patch contained an eofnl change (i.e. the last line either
gained or lost a newline), the oldno and newno line number values
for the lines in the last hunk of the patch were not useful.  This
makes them behave in a more expected manner.

This adds a new line origin constant for the special line that
is used when both files end without a newline.

In the course of writing the tests for this, I was having problems
with modifying a file but not having diff notice because it was
the same size and modified less than one second from the start of
the test, so I decided to start working on nanosecond timestamp
support.  This commit doesn't contain the nanosecond support, but
it contains the reorganization of maybe_modified and the hooks so
that if the nanosecond data were being read by stat() (or rather
being copied by git_index_entry__init_from_stat), then the nsec
would be taken into account.

This new stuff could probably use some more tests, although there
is some amount of it here.

fix git_branch_set_upstream for local branches

Fixed qsort_r() problem when targeting AmigaOS.

We fall back to the libgit2-provided insert sort as done for other
platforms.