This cleans up some places I missed that could hold onto submodule
references and cleans up the way in which the repository cache is
both reloaded and released so that existing submodule references
aren't destroyed inappropriately.

`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored.  This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.

env test needs to deref git_buf's ptr

Refactor the `git_merge` API

settings: use git_buf for returning strings

The default cache size is 256 megabytes, not megabits as claimed in the
docs.

This survived the last round of culling, as the signature is only in the
comments.

Fix wrong assertion

Fixes issue #2196

revwalk: don't try to find merge bases when there can be none

As a way to speed up the cases where we need to hide some commits, we
find out what the merge bases are so we know to stop marking commits as
uninteresting and avoid walking down a potentially very large amount of
commits which we will never see. There are however two oversights in
current code.

The merge-base finding algorithm fails to recognize that if it is only
given one commit, there can be no merge base. It instead walks down the
whole ancestor chain needlessly. Make it return an empty list
immediately in this situation.

The revwalk does not know whether the user has asked to hide any commits
at all. In situation where the user pushes multiple commits but doesn't
hide any, the above fix wouldn't do the trick. Keep track of whether the
user wants to hide any commits and only run the merge-base finding
algorithm when it's needed.

reflog: follow core.logallrefupdates

On bare by default, or when core.logallrefupdates is false, we must not
write the reflog.

Reflog all the way

Given HEAD -> master -> foo, when updating foo's reflog we should also
update HEAD's, as it's considered the current branch.

The reflog append function was overzealous in its checking. When passed
an old and new ids, it should not do any checking, but just serialize
the data to a reflog entry.

Remove some duplicated logic.

If the caller wants to update a ref to point to the same target as it
currently has, we should return early and avoid writing to the reflog.

The existing ones lack checking zeroed ids when switching back from an
unborn branch as well as what happens when detaching.

The reflog appending function mistakenly wrote zeros when dealing with a
detached HEAD. This explicitly checks for those situations and fixes
them.

Implement git_merge_base_octopus

Seamless support for NTLM/Kerberos auth on Windows

refs: append to the HEAD reflog when updating the current branch

Add a few projects to the list

When we update the current branch, we must also append to HEAD's reflog
to keep them in sync.

This is a bit of a hack, but as git.git says, it covers 100% of
default cases.

This is not something anybody would ever do; removing HEAD makes the
.git/ directory no longer be a repository, so we wouldn't be expected to
handle such a situation.