Merge branch 'jk/fallthrough'

Many codepaths have been updated to squelch -Wimplicit-fallthrough
warnings from Gcc 7 (which is a good code hygiene).

* jk/fallthrough:
consistently use "fallthrough" comments in switches
curl_trace(): eliminate switch fallthrough
test-line-buffer: simplify command parsing

diff --cc builtin/cat-file.c
Simple merge

diff --cc config.c
Simple merge

diff --cc read-cache.c
Simple merge