diff_generate: fix unsetting diff flags

Use the same cert checking payload in WinHTTP

The macro `DIFF_FLAG_SET` can be used to set or unset a flag by
modifying the diff's bitmask. While the case of setting the flag is
handled correctly, the case of unsetting the flag was not. Instead of
inverting the flags, we are inverting the value which is used to decide
whether we want to set or unset the bits.

The value being used here is a simple `bool` which is `false`. As that
is being uplifted to `int` when getting the bitwise-complement, we will
end up retaining all bits inside of the bitmask. As that's only ever
used to set `GIT_DIFF_IGNORE_CASE`, we were actually always ignoring
case for generated diffs.

Fix that by instead getting the bitwise-complement of `FLAG`, not `VAL`.

In commit 9be638ec (git_diff_generated: abstract generated diffs,
2016-04-19), the code for generated diffs was moved out of the generic
"diff.c" and instead into its own module. During that conversion, it was
forgotten to remove the macros `DIFF_FLAG_IS_SET`, `DIFF_FLAG_ISNT_SET`
and `DIFF_FLAG_SET`, which are now only used in "diff_generated.c".

Remove those macros now.

CONTRIBUTING: add documentation of our commit message style

Include git2/worktree.h in git2.h

I'm not sure if worktree.h was intentionally left out of git2.h. Looks like an oversight since it is in fact documented.

Getting started README improvements

Introduce a simple FAQ for common errors for people getting started with
the project.

Since we recommend `ctest -V`, it's not clear why somebody would want to
run `libgit2_clar`.  Indicate that it's helpful when running individual
tests or suites.

Provide a very simple quick start paragraph that highlights how easy it
is to get started, and points people toward common problems.

Users should not be advised to use the VS command prompt; instead, they
should let cmake find their Visual Studio installation.

Suggest that users run `ctest -V` instead of `make test` when getting
started.  `ctest -V` is superior over alternatives as:

1. Unlike `make test`, it gives output.  Users getting started with
   the library believe that it is hung.
2. `ctest -V` shows verbose output; showing suite names is helpful for
   giving users more feedback immediately.

While we try to conform to a certain commit message style, this style
has never been documented anywhere. Document it such that new
contributors do not have to go through another needless iteration of
their pull requests just to fix up commit messages.

Gather the reflog entry content tests

diff: expose the "indent heuristic" in the diff options

We default to off, but we might want to consider changing `GIT_DIFF_NORMAL` to
include it.

refcount: make refcounting conform to aliasing rules

Strict aliasing rules dictate that for most data types, you are not
allowed to cast them to another data type and then access the casted
pointers. While this works just fine for most compilers, technically we
end up in undefined behaviour when we hurt that rule.

Our current refcounting code makes heavy use of casting and thus
violates that rule. While we didn't have any problems with that code,
Travis started spitting out a lot of warnings due to a change in their
toolchain. In the refcounting case, the code is also easy to fix:
as all refcounting-statements are actually macros, we can just access
the `rc` field directly instead of casting.

There are two outliers in our code where that doesn't work. Both the
`git_diff` and `git_patch` structures have specializations for generated
and parsed diffs/patches, which directly inherit from them. Because of
that, the refcounting code is only part of the base structure and not of
the children themselves. We can help that by instead passing their base
into `GIT_REFCOUNT_INC`, though.

add test status codes legend to the 'Started' test trace

motivation: (for someone new to the tests) it's puzzling to find the odd 'S' interspersed in the test output

proposed alternative test output (extract):

$ cmake --build . && ./libgit2_clar -srepo -v
...
Loaded 340 suites:
Started (test status codes: OK='.' FAILURE='F' SKIPPED='S')

repo::config...
repo::discover..........
repo::env.
repo::getters...
repo::hashfile..
repo::head......................
repo::headtree....
repo::init.........................S
repo::message..
repo::new..
repo::open.............
repo::pathspec..........
repo::reservedname.....
repo::setters.....
repo::shallow....
repo::state.............

s/Init/Index comment tweak for test_repo_init__init_with_initial_commit

signature: distinguish +0000 and -0000 UTC offsets

Git considers '-0000' a valid offset for signature lines.  They need to
be treated as _not_ equal to a '+0000' signature offset.  Parsing a
signature line stores the offset in a signed integer which does not
distinguish between `+0` and `-0`.

This patch adds an additional flag `sign` to the `git_time` in the
`signature` object which is populated with the sign of the offset.  In
addition to exposing this information to the user, this information is
also used to compare signatures.

/cc @pks-t @ethomson

Fix clar to verify command line arguments before execute

Common parser interface

When executing `libgit2_clar -smerge -invalid_option`, it will first execute
the merge test suite and afterwards output help because of the invalid option.

With this changa, it verifies all options before execute. If there are any
invalid options, it will output help and exit without actually executing
the test suites.

cmake: Allow user to select bundled zlib

As the config parser is now cleanly separated from the config file code,
we can easily refactor the code and make use of the common parser
module. This removes quite a lot of duplicated functionality previously
used for handling the actual parser state and replaces it with the
generic interface provided by the parser context.

The configuration file code grew quite big and intermingles both actual
configuration logic as well as the parsing logic of the configuration
syntax. This makes it hard to refactor the parsing logic on its own and
convert it to make use of our new parsing context module.

Refactor the code and split it up into two parts. The config file code
will only handle actual handling of configuration files, includes and
writing new files. The newly created config parser module is then only
responsible for parsing the actual contents of a configuration file,
leaving everything else to callbacks provided to its provided function
`git_config_parse`.

Upon initializing the parser context, we do not currently initialize the
current line, line length and line number. Do so in order to make the
interface easier to use and more obvious for future consumers of the
parsing API.