8314543: gitattributes: make diffs easier to read
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
