Add annotated versions of ref-modying functions

[WIP] Smarter pack-building

Don't ask for a stream's certificate unless it's encrypted

Fix git_submodule_sync writing URL to wrong key.

When the user has a certificate check callback set, we still have to
check whether the stream we're using is even capable of providing a
certificate.

In the case of an unencrypted certificate, do not ask for it from the
stream, and do not call the callback.

When we have an HTTP stream and have set the certificatre check
callback, we currently fail as we ask the unencrypted stream for its
certificate.

This also brings the soft-reset tests back to life. The function name
was missing an underscore, meaning they had not been running.

This extra constructor will be useful for the annotated versions of
ref-modifying functions, as it allows us to create a commit with the
extended sha syntax which was used to retrieve it.

We do not always want to put the id directly into the reflog, but we
want to speicfy what a user typed. For this use-case we provide
annotated version of a few functions which let the caller specify what
user-friendly name was used when asking for the operation.

Avoid retaining / releasing the index more than necessary when GIT_DIFF_...

It turns out that erroring out on duplicate commits is the right thing
to do, but git was not hitting the bug on the server-side.

Bring back a descriptive error message in case of duplicate entries and
error out.

If a packfile includes duplicate objects, we can choose to use the
secon copy instead of the first by using the same logic as if it were
the first.

Change the error condition from 0 to -1, which indicates a bad resize,
and set the OOM message in that case.

This does mean we will leak the first copy of the object. We can deal
with that later, but making fetches work is more important.

While this is not even close to a fix, we can at least set an error
message so we know which error we are facing. Up to know we just
returned an error without a message.

This was but down to 5 when GitHub made a change to their server which
made them stop honouring the include-tag request.

This has recently been corrected, so we can bring it back up to six.

local: create pack with multiple threads

Currently git_submodule_sync writes the submodule's URL to the
key 'branch.<REMOTE_NAME>.remote' while the reference
implementation of `git submodule sync` writes to
'remote.<REMOTE_NAME>.url', which is the intended behavior
according to git-submodule(1).

The default behaviour for the packbuilder is to perform the work in a
single thread, which is fine for the public API, but we currently have
no way for a user to determine the number of threads to use when
creating the packfile, which makes our clone behaviour over the
filesystem quite a bit slower than what git offers.

This is a very particular scenario, in which we avoid spawning git by
being ourselves the server-side, so it's probably ok to auto-set the
threading, as the upload-pack process would do if we were talking to
git.

refspec: report errors when parsing an invalid refspec

"In-memory" repos are bare by default

Currently we use the most naïve and inefficient method for figuring out
which objects to send to the remote whereby we end up trying to insert
subdirs which have not changed multiple times.

Instead, make use of the packbuilder's built-in more efficient method
which uses the walk to feed the object list and avoids inserting an
object and its descendants.

Most use-cases for the object packer communicate in terms of commits
which each side has. We already have an object to specify this
relationship between commits, namely git_revwalk.

By knowing which commits we want to pack and which the other side
already has, we can perform similar optimisations to git, by marking
each tree as interesting or uninteresting only once, and not sending
those trees which we know the other side has.

Keep the definitions in the headers, while putting the declarations in
the C files. Putting the function definitions in headers causes
them to be duplicated if you include two headers with them.

Revert "fastfail" from the mingw PR

If a refspec could not be parsed, the git_refspec__parse function would
return an error value but would not provide additional error information
for the callers. This commit amends that function to set a more useful
error message.

This reverts commit 8008ab6a.

This reverts commit ba6c53b9.

Fixes

RFC: add mingw to appveyor matrix

refdb: use the same id for old and new when renaming a reference

When we rename a reference, we want the old and new ids to be the same
one (as we did not change it). The normal code path looks up the old id
from the current value of the brtanch, but by the time we look it up, it
does not exist anymore and thus we write a zero id.

Pass the old id explicitly instead.

Filter relative paths

branch: fix generated reflog message upon creation