Ensure that a `.gitattributes` file that is deeper in the tree is
honored, not just an attributes file at the root.

Test that we can optionally ignore system attributes when filtering a
blob.

By default, `/etc/gitattributes` (or the system equivalent) is read to
provide attributes.  Ensure that, by default, this is read when
filtering blobs.

Move users of `git_blob_filtered_content` to `git_blob_filter`.

Add a LF->CRLF conversion test to the blob filter.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

Move to the `git_error` name in the internal API for error-related
functions.

This is a cherry-pick of the tests from the following commits:

core.autocrlf=true and core.safecrlf=true did not fail on LF-only file as vanilla git does
Adding a CRLF-file with core.autocrlf=input and core.safecrlf=true does not fail as with vanilla git
Make files with #CR!=#CRLF not fail with core.safecrlf=true

Reported-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Signed-off-by: Sven Strickroth <email@cs-ware.de>

Use the new object_type enumeration names within the codebase.