Nothing should be hidden and this shouldn't bother the merge base
calculation.

There is no need walk down the parents of a merge base to mark them as
uninteresting because we'll never see them. Calculate the merge bases
in prepare_walk() so mark_uninteresting() can stop at a merge base
instead of walking all the way to the root.

It's implemented in revwalk.c so it has access to the revision
walker's commit cache and related functions. The algorithm is the one
used by git, modified so it fits better with the library's functions.

The code was already there, so factor it out and let users push an OID
by giving it a reference name. Only refs to commits are
supported. Annotated tags will throw an error.

This is Git yo. You can fetch stuff from the history if you need it.

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.