Completion related fixes for clang-include-fixer.el.

1. Quitting inside a process sentinel is not allowed, but the sentinel invokes
completion, where the user is free to hit C-g. By wrapping the call in
with-local-quit, the process sentinel invocation can finish without triggering
an error
2. Invoke completing-read instead of ido-completing-read, since this may
interfere with user customizations to completing-read-function. The user should
use something like ido-ubiquitous if ido completion is wanted
3. Compare the string returned from completion with string=, since it may be a
copy of the original string in the collection

Patch by Torsten Marek.

llvm-svn: 295818