Bug 1411004 - ./mach clang-format: Better handling of relative paths r=mystor=mystor

For example, this is addressing the issue:
./mach clang-format -p ./mfbt/

MozReview-Commit-ID: Le8mPTOEfA7

tools/mach_commands.py

@@ -304,7 +304,8 @@ class FormatProvider(MachCommandBase):
             # Remove comments and empty lines
             if line.startswith('#') or len(line.strip()) == 0:
                 continue
-            ignored_dir.append(line.rstrip())
+             # The regexp is to make sure we are managing relative paths
+            ignored_dir.append("^[\./]*" + line.rstrip())
 
         # Generates the list of regexp
         ignored_dir_re = '(%s)' % '|'.join(ignored_dir)