Use `git_buf_decode_percent` so that we can avoid allocating a temporary
buffer.

CMake: make HTTPS support more generic

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

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

After calling `libssh2_init`, we need to clean up after the library by
executing `libssh2_exit` as soon as we exit. Register a shutdown handler
to do so which simply calls `libssh2_exit`. This fixes several memory
leaks.

We unconditionally return success when initializing libssh2, regardless
of whether `libgssh2_init` signals success or an error. Fix this by
checking its return code.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

Support for ssh+git and git+ssh protocols