Merge branch 'sg/travis-specific-cc'

The travis CI scripts have been corrected to build Git with the compiler(s) of our choice. * sg/travis-specific-cc: travis-ci: build with the right compiler travis-ci: switch to Xcode 10.1 macOS image travis-ci: don't be '--quiet' when running the tests .gitignore: ignore external debug symbols from GCC on macOS