contrib: add new examples area and start it out with a config for `hg fix`

This is the configuration contributors should use for our C/C++
code. I expect to expand this file as we get more automated formatter
oversight.

Differential Revision: https://phab.mercurial-scm.org/D6872

contrib/examples/fix.hgrc

@@ -0,0 +1,3 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"