Running Shfmt only works with saved copy of script

[andrewimeson]

Shfmt silently eats changes in the buffer that have not been written to the file.

Steps to Reproduce:

1. Create a new file vim shfmt-test.bash
2. Put initial contents into the file, and write it with :w

   #!/usr/bin/env bash
   # Reproducce vim-shfmt issue
   for bacon in "turkey" "pig"; do
   echo $bacon | grep -q "pig" &&
       echo "$bacon bacon is real bacon" ||
       echo "$bacon bacon is not real bacon"
   done

3. Run :Shfmt, get the properly reformatted script

   #!/usr/bin/env bash
   # Reproducce vim-shfmt issue
   for bacon in "turkey" "pig"; do
   echo $bacon | grep -q "pig" &&
   echo "$bacon bacon is real bacon" ||
   echo "$bacon bacon is not real bacon"
   done

4. Make some more edits, but do not write them to the file

   
   #!/usr/bin/env bash
   # Reproducce vim-shfmt issue
   for bacon in "turkey" "pig"; do
   echo $bacon | grep -q "pig" &&
   echo "$bacon bacon is real bacon" ||
   echo "$bacon bacon is not real bacon"
   done

echo "really important info that I forgot to add:" echo "bacon goes really well on pizza"

4. Run `:Shfmt` and watch the unwritten edits disappear
```bash
#!/usr/bin/env bash
# Reproducce vim-shfmt issue
for bacon in "turkey" "pig"; do
  echo $bacon | grep -q "pig" &&
    echo "$bacon bacon is real bacon" ||
    echo "$bacon bacon is not real bacon"
done

Environment: macOS 10.14.2 vim 8.1 (from Homebrew) vim-shfmt from commit 5f1fe6a with .vimrc options:

let g:shfmt_extra_args = '-i 2 -ci -s'

[alpayne]

vim-shfmt is indeed running against the last saved file instead of running as a filter. I've submitted a pull request with changes to have it run as a filter instead.