template: Add TEST_USE_REAL_URL to fix Travis

See the comments for `TEST_USE_REAL_URL` in `tests/template.bats` as
well as:

https://travis-ci.org/mbland/go-script-bash/jobs/200488089
https://travis-ci.org/mbland/go-script-bash/jobs/200488076

Author: mbland

tests/template.bats

@@ -2,9 +2,33 @@
 
 load environment
 
+# By default, the test will try to clone its own repo to avoid flakiness due to
+# an external dependency. However, doing so causes a failure on Travis, since it
+# uses shallow clones to produce test runs, resulting in the error:
+#
+#   fatal: attempt to fetch/clone from a shallow repository
+#
+# However, since Travis already depends on having a good connection to GitHub,
+# we'll use the real URL. Alternatively, `git` could be stubbed out via
+# `stub_program_in_path` from `_GO_CORE_DIR/lib/bats/helpers`, but the potential
+# for neither flakiness nor complexity seems that great, and this approach
+# provides extra confidence that the mechanism works as advertised.
+#
+# A developer can also run the test locally against the real URL by setting
+# `TEST_USE_REAL_URL` on the command line. The value of `GO_CORE_URL` is
+# subsequently displayed in the name of the test case to validate which repo is
+# being used during the test run.
+TEST_USE_REAL_URL="${TEST_USE_REAL_URL:-$TRAVIS}"
+GO_CORE_URL="${TEST_USE_REAL_URL:+$_GO_CORE_URL}"
+GO_CORE_URL="${GO_CORE_URL:-$_GO_CORE_DIR}"
+
 setup() {
   test_filter
   mkdir "$TEST_GO_ROOTDIR"
+
+  if [[ -e "$GO_CORE_URL/.git/shallow" ]]; then
+    skip "Can't clone shallow repositories"
+  fi
 }
 
 teardown() {
@@ -29,7 +53,7 @@ create_template_script() {
   chmod 700 "$TEST_GO_ROOTDIR/go-template"
 }
 
-@test "$SUITE: clone the go-script-bash repository" {
+@test "$SUITE: clone the go-script-bash repository from $GO_CORE_URL" {
   create_template_script "$_GO_CORE_DIR" "$_GO_CORE_VERSION"
   run "$TEST_GO_ROOTDIR/go-template"