lfs: generate a large file by using `python` instead of yes | head

yes(1) on some systems (like gcc112) feels compelled to inform you of
broken pipes, such as those triggered by head(1). This works around
the problem portably.

tests/test-lfs-largefiles.t

@@ -199,7 +199,7 b' largefiles since it hooks cmdutil.add() '
 commit.  By the time the commit occurs, the tracked file is smaller than the
 threshold (assuming it is > 41, so the standins don't become lfs objects).
 
-  $ yes | head -n 1048576 > large_by_size.bin
+  $ $PYTHON -c 'import sys ; sys.stdout.write("y\n" * 1048576)' > large_by_size.bin
   $ hg --config largefiles.minsize=1 ci -Am 'large by size'
   adding large_by_size.bin as a largefile
   $ hg manifest