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

diff --combined .gitignore
index 6fd61c7272fb460c50cc0b1e3a08ce97a6cf912f,a9db568712373b7898dae968ca8ce7ce99062dc2..7374587f9df901cba0a6990ad48fac17f8a5be57
--- 1/.gitignore
--- 2/.gitignore
+++ b/.gitignore
@@@ -1,4 -1,3 +1,4 @@@
 +/fuzz-commit-graph
  /fuzz_corpora
  /fuzz-pack-headers
  /fuzz-pack-idx
@@@ -125,6 -124,7 +125,6 @@@
  /git-rebase--am
  /git-rebase--common
  /git-rebase--interactive
 -/git-rebase--merge
  /git-rebase--preserve-merges
  /git-receive-pack
  /git-reflog
@@@ -229,3 -229,4 +229,4 @@@
  *.pdb
  /Debug/
  /Release/
+ *.dSYM