gitlab_git 10.6.4 relies on Rugged marking blobs as binary or not,
instead of relying on Linguist. Linguist in turn would mark text blobs
as binary whenever they would contain byte sequences that could not be
encoded using UTF-8.

However, marking such blobs as binary is not correct. If one pushes a
Markdown document with invalid character sequences it's still a text
based Markdown document and not some random binary blob.

This commit overwrites Blob#data so it automatically converts text-based
content to UTF-8 (the encoding we use everywhere else) while taking care
of replacing any invalid sequences with the UTF-8 replacement character.
The data of binary blobs is left as-is.