cmdutil: simplify the dirty check in howtocontinue()
This is equivalent to the previous code. But it seems to me that if the user is
going to be prompted that a commit is needed, missing files should be ignored,
but branch and merge changes shouldn't be.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true