formatting: enforce clang-format on all .cc files as well as .c files

As part of my upcoming fuzzer patch we're going to grow some .cc
files. They're not part of the release build (they're only used for
fuzzing, which requires clang 6 which in turn implies a working C++
compiler), so I'm not terribly worried about growing some C++ files.

Differential Revision: https://phab.mercurial-scm.org/D1874

Makefile

@@ -124,7 +124,7 b' check-code:'
 
 format-c:
 	clang-format --style file -i \
-	  `hg files 'set:(**.c or **.h) and not "listfile:contrib/clang-format-blacklist"'`
+	  `hg files 'set:(**.c or **.cc or **.h) and not "listfile:contrib/clang-format-blacklist"'`
 
 update-pot: i18n/hg.pot
 

tests/test-check-clang-format.t

@@ -3,7 +3,7 b''
   $ . "$TESTDIR/helpers-testrepo.sh"
 
   $ cd "$TESTDIR"/..
-  $ for f in `testrepohg files 'set:(**.c or **.h) and not "listfile:contrib/clang-format-blacklist"'` ; do
+  $ for f in `testrepohg files 'set:(**.c or **.cc or **.h) and not "listfile:contrib/clang-format-blacklist"'` ; do
   >   clang-format --style file $f > $f.formatted
   >   cmp $f $f.formatted || diff -u $f $f.formatted
   >   rm $f.formatted