gitk: Highlight only when search type is "containing:".

gitk: Highlight only when search type is "containing:".

When the search type is "touching paths" or "adding/removing string",
it's not very useful to highlight instances of the search string in
the commit message, headline or author name, so this disables the
highlighting in those cases.

This was suggested by Mark Burton <redacted>, but the
implementation is different to his patch, which tested $gdttype at
each place where $markingmatches was tested.

Signed-off-by: Paul Mackerras <redacted>