Depending on whether the path we want to look up an attribute for is a
file or a directory, the fnmatch function will be called with different
flags. Because of this, we have to first stat(3) the path to determine
whether it is a file or directory in `git_attr_path__init`. This is
wasteful though in bare repositories, where we can already be assured
that the path will never exist at all due to there being no worktree. In
this case, we will execute an unnecessary syscall, which might be
noticeable on networked file systems.

What happens right now is that we always pass the `GIT_DIR_FLAG_UNKOWN`
flag to `git_attr_path__init`, which causes it to `stat` the file itself
to determine its type. As it is calling `git_path_isdir` on the path,
which will always return `false` in case the path does not exist, we end
up with the path always being treated as a file in case of a bare
repository. As such, we can just check the bare-repository case in all
callers and then pass in `GIT_DIR_FLAG_FALSE` ourselves, avoiding the
need to `stat`. While this may not always be correct, it at least is no
different from our current behavior.

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.

Some implementation files were missing the license headers. This commit
adds them.

The recent introduction of the commondir variable of a repository
requires callers to distinguish whether their files are part of
the dot-git directory or the common directory shared between
multpile worktrees. In order to take the burden from callers and
unify knowledge on which files reside where, the
`git_repository_item_path` function has been introduced which
encapsulate this knowledge.

Modify most existing callers of `git_repository_path` to use
`git_repository_item_path` instead, thus making them implicitly
aware of the common directory.

Minimizing the number directory and file opens, minimizes the amount of IO thus reducing the overall cost of performing ignore operations.

Provide a convenience function that creates a buffer that can be provided
to callers but will not be freed via `git_buf_free`, so the buffer
creator maintains the allocation lifecycle of the buffer's contents.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.