Basic branch management API

Fix compilation warning

Allow references to bear non-7bit-ascii names on Windows

Rewrite status using diff

There was a bug in git_buf_join_n when the contents of the
original buffer were joined into itself and the realloc
moved the pointer to the original buffer.

This adds support for a bunch of core.* settings that affect
diff and status, plus fixes up some incorrect implementations
of those settings from before.  Also, this cleans up the
handling of config settings in the new submodules code and
in the old attrs/ignore code.

When processing status for a newly checked out repo, it is
possible that there will be submodules that have not yet been
initialized.  The only way to distinguish these from untracked
directories is to have some knowledge of submodules.  This
commit adds a new submodule API which, given a name or path,
can determine if it appears to be a submodule and can give
information about the submodule.

I decided that the COITERATE macro was, in the end causing
more confusion that it would save and decided just to write
out the loops that I needed for parallel diff list iteration.
It is not that much code and this just feels less obfuscated.

There was an error in the tree iterator where it would
delete two tree levels instead of just one when popping
up a tree level.  Unfortunately the test data for the
tree iterator did not have any deep trees with subtrees
in the middle of the tree items, so this problem went
unnoticed.  This contains the 1-line fix plus new test
data and tests that reveal the issue.

This gives `git_status_foreach()` back its old behavior of
emulating the "--untracked=all" behavior of git.  You can
get any of the various --untracked options by passing flags
to `git_status_foreach_ext()` but the basic version will
keep the behavior it has always had.

This fixes the bug that @nulltoken found (thank you!) where
if there were untracked directories alphabetically after the
last tracked item, the diff implementation would deref a NULL
pointer.

The fix involved the code which decides if it is necessary
to recurse into a directory in the working dir, so it was
easy to add a new option `GIT_STATUS_OPT_RECURSE_UNTRACKED_DIRS`
to control if the contents of untracked directories should be
included in status.

test helpers: Fix file close on error

This finishes up the migration of remaining tests from
tests/t18-status.c over the tests-clar/status/worktree.c.

This "fixes" the broken t18 status tests to accurately reflect
the new behavior for "created" untracked subdirectories.  See
discussion in the PR for more details.

This also contains the submodules unit test that I forgot to
git add, and ports most of the t18-status.c tests to clar (still
missing a couple of the git_status_file() single file tests).

This adds support for roughly-right tracking of submodules
(although it does not recurse into submodules to detect
internal modifications a la core git), and it adds support
for including unmodified files in diff iteration if requested.

This is a work in progress.  This adds two new sets of tests,
the issue_592 tests from @nulltoken's pull request #601 and
some new tests for submodules.  The submodule tests still have
issues where the status is not reported correctly.  That needs
to be fixed before merge.

This is an initial reimplementation of status using diff a la
the way that core git does it.

More error handling conversions

Cleaned up some other issues.

More files moved to new error handling style.

This includes a few cleanups that came up while converting
these files.

This commit introduces a could new git error classes, including
the catchall class: GITERR_INVALID which I'm using as the class
for invalid and out of range values which are detected at too low
a level of library to use a higher level classification.  For
example, an overflow error in parsing an integer or a bad letter
in parsing an OID string would generate an error in this class.

Also cleaned up some previously converted code that still had
little things to polish.

Fix windows network paths

Fix libgit2/libgit2sharp#125