Since a625b092c (branch: correctly reject refs/heads/{-dash,HEAD},
2017-11-14), which is included in v2.16.0, upstream git refuses to
create branches which are named HEAD to avoid ambiguity with the
symbolic HEAD reference. Adjust our own code to match that behaviour and
reject creating branches names HEAD.

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.

Be sure to clean up looked up references.  Free buffers instead of
merely clearing them.  Use `git__free` instead of `free`.

Previously, we have extracted the logic to find and iterate over all
HEADs of a repository. Use this function in `git_branch_is_checked_out`.

Restrict the ability to delete branches that are checked out in
any linked repository.

Implement a new function that is able to determine if a branch is
checked out in any repository connected to the current
repository. In particular, this is required to check if for a
given repository and branch, there exists any working tree
connected to that repository that is referencing this branch.

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

The code correctly detects that forced creation of a branch on a
nonbare repo should not be able to overwrite a branch which is
the HEAD reference.  But there's no reason to prevent this on
a bare repo, and in fact, git allows this.  I.e.,

   git branch -f master new_sha

works on a bare repo with HEAD set to master.  This change fixes
that problem, and updates tests so that, for this case, both the
bare and nonbare cases are checked for correct behavior.

Differentiate between the ref_name used to create an annotated_commit
(that can subsequently be used to look up the reference) and the
description that we resolved this with (which _cannot_ be looked up).

The description is used for things like reflogs (and may be a ref name,
and ID something that we revparsed to get here), while the ref name must
actually be a reference name, and is used for things like rebase to
return to the initial branch.

Removing a reflog upon ref deletion is something which only some
backends might wish to do. Backends which are database-backed may wish
to archive a reflog, log-based ones may not need to do anything.