+# rename detection options for git-diff and git-diff-tree
+# - default is '-M', with the cost proportional to
+# (number of removed files) * (number of new files).
+# - more costly is '-C' (or '-C', '-M'), with the cost proportional to
+# (number of changed files + number of removed files) * (number of new files)
+# - even more costly is '-C', '--find-copies-harder' with cost
+# (number of files in the original tree) * (number of new files)
+# - one might want to include '-B' option, e.g. '-B', '-M'
+our @diff_opts = ('-M'); # taken from git_commit
+