+my $subdir_prefix = `git rev-parse --show-prefix`;
+chomp($subdir_prefix);
+
+# run in git base directory, so that git-ls-files lists all revisioned files
+chdir "$GIT_DIR/..";
+
+# normalize paths, needed to compare against versioned files and update-index
+# also, this is nicer to end-users by doing ".//a/./b/.//./c" ==> "a/b/c"
+for (@srcArgs, @dstArgs) {
+ # prepend git prefix as we run from base directory
+ $_ = $subdir_prefix.$_;
+ s|^\./||;
+ s|/\./|/| while (m|/\./|);
+ s|//+|/|g;
+ # Also "a/b/../c" ==> "a/c"
+ 1 while (s,(^|/)[^/]+/\.\./,$1,);
+}
+