worktree:  add functions to get name and path

worktree: add ability to create worktree with pre-existing branch

Currently, we always create a new branch after the new worktree's name
when creating a worktree. In some workflows, though, the caller may want
to check out an already existing reference instead of creating a new
one, which is impossible to do right now.

Add a new option `ref` to the options structure for adding worktrees. In
case it is set, a branch and not already checked out by another
worktree, we will re-use this reference instead of creating a new one.

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.