We compute offsets by executing `off |= (*delta++ << 24)` for
multiple constants, where `off` is of type `size_t` and `delta`
is of type `unsigned char`. The usual arithmetic conversions (see
ISO C89 §3.2.1.5 "Usual arithmetic conversions") kick in here,
causing us to promote both operands to `int` and then extending
the result to an `unsigned long` when OR'ing it with `off`.
The integer promotion to `int` may result in wrong size
calculations for big values.

Fix the issue by making the constants `unsigned long`, causing both
operands to be promoted to `unsigned long`.

An object's size is computed by reading the object header's size
field until the most significant bit is not set anymore. To get
the total size, we increase the shift on each iteration and add
the shifted value to the total size.

We read the current value into a variable of type `unsigned
char`, from which we then take all bits except the most
significant bit and shift the result. We will end up with a
maximum shift of 60, but this exceeds the width of the value's
type, resulting in undefined behavior.

Fix the issue by instead reading the values into a variable of
type `unsigned long`, which matches the required width. This is
equivalent to git.git, which uses an `unsigned long` as well.

When `git_repository__cvar` fails we may end up with a
`ignorecase` value of `-1`. As we subsequently check if
`ignorecase` is non-zero, we may end up reporting that data
should be removed when in fact it should not.

Err on the safer side and set `ignorecase = 0` when
`git_repository__cvar` fails.

The `merge_file__xdiff` function checks if either `ours` or
`theirs` is `NULL`. The function is to be called with existing
files, though, and in fact already unconditionally dereferences
both pointers.

Remove the unnecessary check to silence warnings.

cmake: include threading libraries in pkg-config

Include any required threading libraries in our `libgit2.pc`.

Introduce `git_signature_from_buffer`

Fix return value of openssl_read (infinite loop)

Allow users to construct a signature from the type of signature
lines that actually appear in commits.

Annotated commits: differentiate between the ref names and the description

Differentiate between the ref_name used to create an annotated_commit
(that can subsequently be used to look up the reference) and the
description that we resolved this with (which _cannot_ be looked up).

The description is used for things like reflogs (and may be a ref name,
and ID something that we revparsed to get here), while the ref name must
actually be a reference name, and is used for things like rebase to
return to the initial branch.

openssl_read should return -1 in case of error.

SSL_read returns values <= 0 in case of error.

A return value of 0 can lead to an infinite loop, so the return value
of ssl_set_error will be returned if SSL_read is not successful (analog
to openssl_write).

Test that we can properly abort a rebase when it is initialized by a
revspec.  This ensures that we do not conflate revspecs and refnames.

Remove traces of `git_blob_create_fromchunks`

Allow creating copies of `git_reference` objects.

Rebase improvements with IDs

tag: ignore extra header fields

While no extra header fields are defined for tags, git accepts them by
ignoring them and continuing the search for the message. There are a few
tags like this in the wild which git parses just fine, so we should do
the same.

⚡ some warnings

When rebasing with IDs, we do not return to the `branch`,
we remain in a detached HEAD state.

When `init`ing a rebase from a detached HEAD, be sure to remember
that we were in a detached HEAD state so that we can correctly
`abort` the object that we just created.

Instead of `open`ing a rebase and `abort`ing that, test that we can
`abort` a rebase that has just begun with `init`.

Proxy configuration

We were downloading the jar from within an block which only runs for
MSVC. Move the download to the start of the test so it gets downloaded
for both.

Improve star-star matching

Instead of threading the state down to the larger loop, let's have the
loop where we detect the double star so each of them are easier to read.

In order to match the star-star, we disable the flag that's looking for
a single path element, but that leads to searching for the pattern in
the middle of elements in the input string.

Mark when we're handing a star-star so we jump over the elements in our
attempt to match the part of the pattern that comes after the star-star.

While here, tighten up the check so we don't allow invalid rules
through.

Running clar directly on appveyor makes it think the command returned
failure, so it stops the tests. Running it via ctest lets it go through.

We leave this up to the scheme in the url field. The type should only
tell us about whether we want a proxy and whether we want to auto-detect
it.

It takes a bit for the propxy to get ready to accept connections, so
start it before the build so we can be reasonably sure that it's going
to be ready in time.