CMake: make HTTPS support more generic

travis: put clar's sandbox in a ramdisk on macOS

appveyor: build examples

The getline(3) function call is not part of ISO C and, most importantly,
it is not implemented on Microsoft Windows platforms. As our networking
example code makes use of getline, this breaks builds on MSVC and MinGW.
As this code wasn't built prior to the previous commit, this was never
noticed.

Fix the error by instead implementing a `readline` function, which
simply reads the password from stdin until it reads a newline
character.

By default, CMake will not build our examples directory. As we do not
instruct either the MinGW or MSVC builds on AppVeyor to enable building
these examples, we cannot verify that those examples at least build on
Windows systems.

Fix that by passing `-DBUILD_EXAMPLES=ON` to AppVeyor's CMake
invocation.

ignore spaces in .gitignore files

Conditional includes

The macOS tests are by far the slowest right now. This attempts to remedy the
situation somewhat by asking clar to put its test data on a ramdisk.

We put our repository in the temporary directory which makes macOS map the path
into a virtual path. `realpath(3)` can resolve it and we do so during repository
opening, but that makes its path have a different prefix from the sandbox path
clar thinks we have.

Resolve the sandbox path before putting it into the test config files so the
paths match as expected.

travis: grab pgp key from www.edwardthomson.com

Preserve the input casing when writing config files

Getting the key from the MIT keyserver is surprisingly unreliable.
Try getting it from my website instead...

For sections we will still use the existing one even if the case disagrees, but
the variable always gets written with the case given by the caller.

While most parts of a configuration key are case-insensitive, we should still be
case-preserving and write down whatever string the caller provided.

example-log: add support for --log-size

Implement a diff indent heuristic

Ask for SSH credentials again when passphrase is wrong

examples: remove Makefile

This allows us to only link against CoreFoundation when using the SecureTransport backend

It defaults to ON, e.g. "pick whatever default is appropriate for the platform".
It accepts one of SecureTransport, OpenSSL, WinHTTP, or OFF.
It errors if the backend library couldn't be found.

When trying to decode the private key it looks like LibSSH2 returns a
LIBSSH2_ERROR_PUBLICKEY_UNVERIFIED when the passphrase is incorrect.

Back in the days when libgit2 was still young, libgit2 was using plain
Makefiles as build infrastructure. We later changed that to instead use
the CMake build system to make cross-platform development easier. In the
process, we forgot to remove the Makefile from our examples directory,
which is fixed by this commit here. Furthermore, remove the accompanying
.gitignore file, which ignores build outputs. As we do out-of-tree
builds only nowadays, no output is generated in that directory anymore.

cmake: use project-relative binary and source directories