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.

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

When the user passes in a diff which has no repository associated
we may call `git_config__get_int_force` with a NULL-pointer
configuration. Even though `git_config__get_int_force` is
designed to swallow errors, it is not intended to be called with
a NULL pointer configuration.

Fix the issue by only calling `git_config__get_int_force` only
when configuration could be retrieved from the repository.

The `normalize_find_opts` function in theory allows for the
incoming diff to have no repository. When the caller does not
pass in diff find options or if the GIT_DIFF_FIND_BY_CONFIG value
is set, though, we try to derive the configuration from the
diff's repository configuration without first verifying that the
repository is actually set to a non-NULL value.

Fix this issue by explicitly checking if the repository is set
and if it is not, fall back to a default value of
GIT_DIFF_FIND_RENAMES.

When stashing the workdir tree, examine the index as well.  Using
a mechanism similar to `git_diff_tree_to_workdir_with_index`
allows us to determine that a file was added in the index and
subsequently modified in the working directory.  Without examining
the index, we would erroneously believe that this file was
untracked and fail to include it in the working directory tree.

Use a slightly modified `git_diff_tree_to_workdir_with_index` in
order to avoid some of the behavior custom to `git diff`.  In
particular, be sure to include the working directory side of a
file when it was deleted in the index.

Drop `git_diff__merge_like_cgit_reversed`, since it's a copy and
paste mess of slightly incompatible changes.