Unlike ignore files, gitattribute files can have flexible whitespace at
the beginning of the line.  Ensure that by adding new ignore rules that
we have not impeded correct parsing of attribute files.

Previously, we would clobber any extension-specific error message with
an "extension is truncated" message. This makes `read_extension`
correctly preserve those errors, takes responsibility for truncation
errors, and adds a new message with the actual extension signature for
unsupported mandatory extensions.

This changes that file to use UNIX line-endings, which makes sense since
this is a UNIXy file.

Use the crlf data scripts to produce a corpus of known-good data in
"git" format (aka ODB format) from a variety of files with different
line endings.  `git` created these files running `git add` to stage the
contents then extracting the data from the repository.

We'll use these to ensure that we create identical contents when we add
files into the index.

Re-use the existing crlf data generation script for creating the to-odb
dataset.  Also, store the actual file contents instead of the ID so that
we can identify differences instead of detecting that differences exist.

Include a shell script that will generate the expected data of OIDs and
failures for calling git.git to capture its output as a test resource.

Right now, there is no need to differentiate different systems as git behaves
the same on all systems IIRC.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Move the crlf_data folders reponsible for holding the state of the
filters going into the working directory to "to_workdir" variations of
the folder name to accommodate future growth into the "to odb" filter
variation.  Update the script to create these new folders as appopriate.

Patch application need not be on an unmodified file; applying to an
already changed file is supported provided the patch still applies
cleanly.  Add tests that modifies the contents of a file then applies
the patch and ensures that the patch applies cleanly, and the original
changes are also kept.

The testrepo test fixture has an index file that's damaged, missing an
object.  The index previously had an entry of `src/index.c` with id
3161df8cbf3a006b4ef85be6497a0ea6bde98541, but that object was missing in
the repository.  This commit adds an object to the repository and
updates the index to use that existing blob.

Similarly, the index has an entry for `readme` with an id of
97328ac7e3bd0bcd3900cb3e7a624d71dd0df888.  This can be restored from
other test repositories.

With these fixed, now the write tree from index tests can pass since they
validate object existence.