gdb: remove tailing newlines from index_cache_debug calls

I noticed that most of the calls to index_cache_debug include a
trailing newline.  As the new debug mechanism already adds a newline,
that means all of these debug calls result in a blank line being
printed, which I think is a mistake.

Remove all the trailing newlines.

I also reformatted one of the index_cache_debug where a string will
now fit onto a single line.

Unless 'set debug index-cache on' is used, there should be no visible
change in output after this commit.