Plug git_merge() related leaks

merge!

Fix leaks

branch: move from foreach to an iterator

Preserve error messages during file system iterator cleanup

Create a git_branch_iterator type which is equivalent to the foreach but
lets us write loops instead of callbacks.

Since the introduction of git_reference_shorthand(), the added value of
passing the name is reduced.

Propagate ELOCKED error when updating the config

Take umask into account in filebuf_commit

When the filesystem iterator encounters an error with a file, it
returns the error but because of the cleanup code, it was in some
cases erasing the error message.  This uses the giterr_detach API
to make sure that the actual error message is restored after the
cleanup code has been run.

There are a number of cases where it is convenient to be able to
fetch and "claim" the current error string, clearing the error.
This is helpful when you need to call some code that may alter
the error and you want to restore it later on and/or report it via
some other mechanism.

Allow backend consumers to specify file mode

Blame Canada

Relicense examples under CC0

Don't assert in git_checkout_tree

Fix some observed problems with incorrect diffs

Update the explanation to reflect our use of git_status_list_new() and
make the breaks in rocco more meaningful.

Clarify why GIT_STATUS_CURRENT and index_to_workdir don't always imply
each other. Fixes #1740.

We used to move `data_start` forward, which is wrong as that needs to
point to the beginning of the buffer in order to perform size
calculations.

Introduce a `write_start` variable which indicates where we should start
writing from, which is what the `data_start` was being wrongly reused to
be.

The last commit taught git_checkout_tree to actually do something
meaningfull, when treeish was NULL. This lets us rewrite
git_checkout_head to simply call git_checkout_tree without giving it a
treeish.

In git_checkout_tree, the first check tests if either repo or treeish is
NULL and says that eithor of them has to have a valid value. But there
is no code to handle the treeish == NULL case.

So, do something meaningful in that case: use HEAD instead.

Since all 3 checkout APIs perform the same operation with the options,
all of them should use the same const-ness.

preserve windows error numbers as well

This was never really working right because we were checking the
wrong flag and not checking it in all the places that we need to
be checking it.  I finally got around to writing a test and adding
actual support for it.

There were a lot more cases to deal with to make sure that our
merged (i.e. workdir-to-tree-to-index) diffs were matching the
output of core Git.

Sometimes the static initializer for git_diff_options cannot be
used and since setting them to all zeroes doesn't actually work
quite right, this adds a new helper for that situation.

This also adds an explicit new value to the submodule settings
options to be used when those enums need static initialization.