[lmi] git: committing content and mode changes together

[Greg Chicares]

Working with one particular file (in a proprietary repository),
I changed its contents and its mode, thus:
  git update-index --chmod=-x my_file.txt
  sed -i my_file.txt -e's/      //g'
and then committed it
  git commit my_file.txt -m'Remove tabs and executable bit'
but the mode change didn't get committed.

Then I did a 'reset --hard', and split it into two steps:

  git update-index --chmod=-x my_file.txt
  git commit my_file.txt -m'Reset executable bit'

and that step worked, but then when I did this:

  sed -i my_file.txt -e's/      //g'
  git commit my_file.txt -m'Remove tabs'

the mode change was reverted. The output has scrolled away into nirvana,
but IIRC correctly the log said
  old mode 100644
  new mode 100755
and
  git ls-tree HEAD |grep my_file.txt
showed the wrong mode, as did 'ls -l my_file.txt'.

Finally I found something that seemed to work: 'reset --hard', then
  chmod -x my_file.txt
and then repeat the (split) steps above.

Why did this seem so difficult? I guess I must be missing something
fundamental, because usually I can find a stackoverflow article that
helps, but this time I can't. I did find this:

http://stackoverflow.com/questions/1611211/how-do-i-make-git-accept-mode-changes-without-accepting-all-text-changes/1611681#1611681

which shows how to commit content and mode changes separately, but
my problem is the opposite: I want to commit them together. What's
the simple, obvious sequence of steps to do that?