ci/travis: Don't destroy cache during prepare

Use `cp -r` instead of `mv`.
Remove use of `dirname`, that was missed in 10cdf8c286.

closes #5166

ci/before_cache.sh

@@ -4,6 +4,7 @@ set -e
 set -o pipefail
 
 CI_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "${CI_DIR}/common/build.sh"
 source "${CI_DIR}/common/suite.sh"
 
 # Don't cache pip's log and selfcheck.
@@ -15,5 +16,5 @@ if ended_successfully; then
   rm -rf "${HOME}/.cache/nvim-deps"
   mv "${DEPS_BUILD_DIR}" "${HOME}/.cache/nvim-deps"
   touch "${CACHE_MARKER}"
-  echo "Updated third-party dependencies (timestamp: $(stat -c '%y' "${CACHE_MARKER}"))."
+  echo "Updated third-party dependencies (timestamp: $(_stat "${CACHE_MARKER}"))."
 fi

ci/common/build.sh

@@ -1,3 +1,11 @@
+_stat() {
+  if test "${TRAVIS_OS_NAME}" = osx ; then
+    stat -f %Sm "${@}"
+  else
+    stat -c %y "${@}"
+  fi
+}
+
 top_make() {
   echo '================================================================================'
   # Travis has 1.5 virtual cores according to:
@@ -19,19 +27,12 @@ build_deps() {
   fi
 
   rm -rf "${DEPS_BUILD_DIR}"
+  mkdir -p "${DEPS_BUILD_DIR}"
 
   # Use cached dependencies if $CACHE_MARKER exists.
   if test -f "${CACHE_MARKER}" && ! test "${CACHE_ENABLE}" = "false" ; then
-    local statcmd="stat -c '%y'"
-    if test "${TRAVIS_OS_NAME}" = osx ; then
-      statcmd="stat -f '%Sm'"
-    fi
-    echo "Using third-party dependencies from Travis cache (last update: $(${statcmd} "${CACHE_MARKER}"))."
-
-    mkdir -p "$(dirname "${DEPS_BUILD_DIR}")"
-    mv "${HOME}/.cache/nvim-deps" "${DEPS_BUILD_DIR}"
-  else
-    mkdir -p "${DEPS_BUILD_DIR}"
+    echo "Using third-party dependencies from Travis cache (last update: $(_stat "${CACHE_MARKER}"))."
+    cp -r "${HOME}/.cache/nvim-deps" "${DEPS_BUILD_DIR}"
   fi
 
   # Even if we're using cached dependencies, run CMake and make to