Document/improve clang-format check in CI

[PJK]

Current failure looks like this, which is not helpful:

+clang-format -version
clang-format version 5.0.0 (tags/RELEASE_500/final)
+clang-format-8 -version
clang-format version 8.0.1-svn360950-1~exp1~20190517011409.67 (branches/release_80)
+./clang-format.sh
/usr/bin/clang-format-8
+git status
+grep 'nothing to commit, working tree clean'
The command "bash -ex .travis-qemu.sh" exited with 1.

[babelouest]

For the clang-format command, adding the option --verbose will probably be helpful to point out format errors.

Also, you can replace the command

git status | grep "nothing to commit, working tree clean"

with the command

git diff-index --quiet HEAD

This will return 0 if the local files are different from the HEAD.

[PJK]

Fixed in #230