This reverts commit d4dfb1f9.

Depends are not correctly generated anymore.

Otherwise, as Helmut pointed out, we will skip Haskell comments or other lines
during the diff.

Let's document possible exit codes while we are at it.

Closes: #774983

Adding `--text=FILE` to the command line will now write differences in plain
text to the given FILE.

FILE can be `-` for standard output. `--html` also gained support for this.

Based on a patch by Helmut Grohne.

Closes: #778423

This reverts commit da146535.

The exit code is part of the external interface of debbindiff. The code needs
be clear about what the exit code is in a given situation. The reverted change
obfuscated the actual exit code.

Add rpm-common to Build-Depends so that we can "import rpm" during build; python-rpm is not sufficient. This is not required in the binary Depends due to rpm2cpio.

To compare text files we need to know the encoding. So we need
to rely solely on libmagic and not match the extension.

In order to properly read text files, we need to get the encoding.
As they are identified by libmagic, we simply need to pass what appears
in the MIME type to the comparator function. We do so by capturing
part of the regex using named group, and passing the resulting dictionary
to the comparison function.

With a small context large blocks of changes that have a single
or a few equal lines inbetween will be split into multiple
blocks.  By having a larger context, they will stay in one block
and more lines can be trimmed.