Make clang-include-fixer--insert-line work when the difference is on an empty line

`clang-include-fixer--insert-line` has an off-by-one error because it
uses `(goto-char (point-min)) (forward-char chars)`, which is (goto-char
(1+ chars))`. Because of this, when the first difference was on an empty
line (i.e. an include was appended to the block of includes), the
pointer in the `to` buffer would be on the next line.

Also wrapped calls inside another process sentinel inside `with-local-quit`.

Patch by Torsten Marek.

Differential Revision: https://reviews.llvm.org/D30292

llvm-svn: 295988