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

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

Due to our split of CMake files into multiple modules, we had to replace
some uses of the `${CMAKE_CURRENT_SOURCE_DIR}` and
`${CMAKE_CURRENT_BINARY_DIR}` variables and replace them with
`${CMAKE_SOURCE_DIR}` and `${CMAKE_BINARY_DIR}`. This enabled us to
still be able to refer to top-level files when defining build
instructions inside of a subdirectory.

When replacing all variables, it was assumed that the absolute set of
variables is always relative to the current project. But in fact, this
is not the case, as these variables always point to the source and
binary directory as given by the top-levl project. So the change
actually broke the ability to include libgit2 directly as a subproject,
as source files cannot be found anymore.

Fix this by instead using project-specific source and binary directories
with `${libgit2_SOURCE_DIR}` and `${libgit2_BINARY_DIR}`.

Fix compilation for CMake versions 3.0.x where x >= 1

`inline` is not portable enough, and the `xdiff` code doesn't import
the `GIT_INLINE` macro. So introduce a new `XDL_INLINE` macro (with
the same definition as `GIT_INLINE`). Use the new macro to inline two
functions in `xdiffi.c`.

Apparently policy CMP0015 was added in CMake 3.1. With CMake 3.0.2,
the build was failing with

    CMake Error at CMakeLists.txt:18 (CMAKE_POLICY):
      Policy "CMP0051" is not known to this version of CMake.

This patch makes it work.

Cleaned up the PR to address styling issues.

cmake: use static dependencies when building static libgit2

cmake: fix linking in Xcode with object libraries only

transports: smart: fix memory leak when skipping symbolic refs

Next to the "gitdir" conditional for including other configuration
files, there's also a "gitdir/i" conditional. In contrast to the former
one, path matching with "gitdir/i" is done case-insensitively. This
commit implements the case-insensitive condition.

Upstream git.git has implemented the ability to include other
configuration files based on conditions. Right now, this only includes
the ability to include a file based on the gitdir-location of the
repository the currently parsed configuration file belongs to. This
commit implements handling these conditional includes for the
case-sensitive "gitdir" condition.