git-gui: Fix use of hunk tag for non-hunk content.

git-gui: Fix use of hunk tag for non-hunk content.

The hunk tag d_@ lost its blue forground color in "apply color information
from git diff output" (2010-10-22, 8f85599). But this tag was also used
for non-hunk content like untracked file mime types or git submodules.

Introduce a new tag for this type of content which has the blue forground
again.

Signed-off-by: Bert Wesarg <redacted>
Signed-off-by: Pat Thoyts <redacted>