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.

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

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.

Cleaned up the PR to address styling issues.

Later on, we will move detection of required libraries, library
directories as well as include directories into a separate
CMakeLists.txt file inside of the source directory. Obviously, we want
to avoid duplication here regarding these parameters.

To prepare for the split, put the parameters into three variables
LIBGIT2_LIBS, LIBGIT2_LIBDIRS and LIBGIT2_INCLUDES, tracking the
required libraries, linking directory as well as include directories.
These variables can later be exported into the parent scope from inside
of the source build instructions, making them readily available for the
other subdirectories.

The credentials callback reads the username and password via scanf into
fixed-length arrays. While these are simply examples and as such not as
interesting, the unchecked return value of scanf causes GCC to emit
warnings. So while we're busy to shut up GCC, we also fix the possible
overflow of scanf by using getline instead.