web: Use correct result MIME types.
We inherited the incorrect type from C git, which has since fixed this
bug. The C git client doesn't actually check the MIME type, but other
clients, such as JGit, may.
For more details, see:
http://repo.or.cz/w/git.git/commitdiff/8efa5f629efb9a8af48619ee90dee02343e0f19d
http://code.google.com/p/support/issues/detail?id=5576
Change-Id: I66f94c59ef85fcc0a79c339b204cdc1798ed797c