sub changedir_git_toplevel () {
my $toplevel = cmdoutput qw(git rev-parse --show-toplevel);
+ length $toplevel or fail __ <<END;
+not in a git working tree?
+(git rev-parse --show-toplevel produced no output)
+END
chdir $toplevel or fail f_ "chdir toplevel %s: %s\n", $toplevel, $!;
}