fix: correct the clang-format example hgrc so that it actually works

There are three changes here:

*   Remove -i from `command`, it causes fix to eat your file and empty it out
*   Add `set:` to pattern, otherwise this is interpreted as just a glob
*   Switch `listfile:` to `include:`; `listfile:` is relative to the current
    working directory, while `include:` is relative to the repo root. This makes
    it so that you don't receive errors when running outside of the repo root
    about being unable to find the file.

Differential Revision: https://phab.mercurial-scm.org/D7618

contrib/examples/fix.hgrc

@@ -1,6 +1,6 b''
 [fix]
-clang-format:command = clang-format --style file -i
-clang-format:pattern = (**.c or **.cc or **.h) and not "listfile:contrib/clang-format-ignorelist"
+clang-format:command = clang-format --style file
+clang-format:pattern = set:(**.c or **.cc or **.h) and not "include:contrib/clang-format-ignorelist"
 
 rustfmt:command = rustfmt
 rustfmt:pattern = set:**.rs