Git supports special hunk headers for several languages in diff output,
which make it easier to read diffs of files in that language, generated
by Git (git-diff, git-show, `git log -p`, etc). For details, see
`git help gitattributes` or the online documentation.[1]
Add entries to the root .gitattributes file to support showing the hunk
headers for Java, C, C++, Markdown, Shell script, HTML, and CSS. This
makes it easier to read diffs generated by Git.
[1] https://git-scm.com/docs/gitattributes
Reviewed-by: erikj, ksakata
