Don't use `git checkout --unshallow` for full-repo Travis CI builds.

Review Request #10974 — Created March 26, 2020 and submitted — Latest diff uploaded

Information

rbintegrations
release-1.0.x
b974db8...

Reviewers

Some people use git: depth: False in thier Travis CI config, most
commonly because tests may need full access to the repository history.
In this case, we were still including git checkout --unshallow in the
build script, which wasn't fatal but produced ugly warnings. This change
checks for a full-repo checkout in the Travis config and only adds the
checkout if necessary.

Created a Travis integration that included git: depth: False in the
config. Saw that prior to this change, an ugly error was shown in the
build log, and after everything was fine.