hghave: add a check for clang-format

Differential Revision: https://phab.mercurial-scm.org/D1068

tests/hghave.py

@@ -439,6 +439,11 b' def has_pylint():'
                        br"Usage:  pylint",
                        True)
 
+@check("clang-format", "clang-format C code formatter")
+def has_clang_format():
+    return matchoutput("clang-format --help",
+                       r"^OVERVIEW: A tool to format C/C\+\+[^ ]+ code.")
+
 @check("pygments", "Pygments source highlighting library")
 def has_pygments():
     try: