diff-highlight: allow testing with Git 3.0 breaking changes

The diff-highlight (in contrib/) comes with its own test script,
which relies on the initial branch name being 'master'.  This is not
just encoded in the test logic, but in the illustration in the file
that shows the topology of the history.

Force the initial branch name to 'master' to allow it pass.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

contrib/diff-highlight/t/t9400-diff-highlight.sh

@@ -10,6 +10,8 @@ DIFF_HIGHLIGHT="$CURR_DIR"/../diff-highlight
 CW="$(printf "\033[7m")"	# white
 CR="$(printf "\033[27m")"	# reset
 
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 . "$TEST_DIRECTORY"/test-lib.sh
 
 if ! test_have_prereq PERL