Add --root flag used for deriving header guard CPP variable.

By default, the header guard CPP variable is calculated as the relative 
path to the directory that contains .git, .hg, or .svn. When this flag 
is specified, the relative path is calculated from the specified 
directory.

Patch by mazda@chromium.org
CR: https://codereview.appspot.com/8630045/

cpplint/cpplint.py

@@ -136,6 +136,22 @@ Syntax: cpplint.py [--verbose=#] [--output=vs7] [--filter=-x,+y,...]
       the top-level categories like 'build' and 'whitespace' will
       also be printed. If 'detailed' is provided, then a count
       is provided for each category like 'build/class'.
+
+    root=subdir
+      The root directory used for deriving header guard CPP variable.
+      By default, the header guard CPP variable is calculated as the relative
+      path to the directory that contains .git, .hg, or .svn.  When this flag
+      is specified, the relative path is calculated from the specified
+      directory. If the specified directory does not exist, this flag is
+      ignored.
+
+      Examples:
+        Assuing that src/.git exists, the header guard CPP variables for
+        src/chrome/browser/ui/browser.h are:
+
+        No flag => CHROME_BROWSER_UI_BROWSER_H_
+        --root=chrome => BROWSER_UI_BROWSER_H_
+        --root=chrome/browser => UI_BROWSER_H_
 """
 
 # We categorize each error message we print.  Here are the categories.
@@ -336,6 +352,10 @@ _RE_SUPPRESSION = re.compile(r'\bNOLINT\b(\([^)]*\))?')
 # on which those errors are expected and should be suppressed.
 _error_suppressions = {}
 
+# The root directory used for deriving header guard CPP variable.
+# This is set by --root flag.
+_root = None
+
 def ParseNolintSuppressions(filename, raw_line, linenum, error):
   """Updates the global list of error-suppressions.
 
@@ -1102,7 +1122,10 @@ def GetHeaderGuardCPPVariable(filename):
   filename = re.sub(r'/\.flymake/([^/]*)$', r'/\1', filename)
 
   fileinfo = FileInfo(filename)
-  return re.sub(r'[-./\s]', '_', fileinfo.RepositoryName()).upper() + '_'
+  file_path_from_root = fileinfo.RepositoryName()
+  if _root:
+    file_path_from_root = re.sub('^' + _root + os.sep, '', file_path_from_root)
+  return re.sub(r'[-./\s]', '_', file_path_from_root).upper() + '_'
 
 
 def CheckForHeaderGuard(filename, lines, error):
@@ -3933,7 +3956,8 @@ def ParseArguments(args):
   try:
     (opts, filenames) = getopt.getopt(args, '', ['help', 'output=', 'verbose=',
                                                  'counting=',
-                                                 'filter='])
+                                                 'filter=',
+                                                 'root='])
   except getopt.GetoptError:
     PrintUsage('Invalid arguments.')
 
@@ -3959,6 +3983,9 @@ def ParseArguments(args):
       if val not in ('total', 'toplevel', 'detailed'):
         PrintUsage('Valid counting options are total, toplevel, and detailed')
       counting_style = val
+    elif opt == '--root':
+      global _root
+      _root = val
 
   if not filenames:
     PrintUsage('No files were specified.')