lint-hunks: better support for working tree

When run with no arguments, report warnings in the diff between the
working tree and HEAD. With arguments, report warnings in the diff
between the named commit and its parents.

Change-Id: Ie10dcdecb303edf8af51bad645cc11206a1fc26b

tools/lint-hunks.py

diff --git a/tools/lint-hunks.py b/tools/lint-hunks.py
index 312fab1..b15a691 100755
--- a/tools/lint-hunks.py
+++ b/tools/lint-hunks.py
@@ -22,7 +22,7 @@
 
 TOPLEVEL_CMD = ["git", "rev-parse", "--show-toplevel"]
 DIFF_CMD = ["git", "diff"]
-DIFF_INDEX_CMD = ["git", "diff-index", "-u", "--cached", "HEAD", "--"]
+DIFF_INDEX_CMD = ["git", "diff-index", "-u", "HEAD", "--"]
 SHOW_CMD = ["git", "show"]
 CPPLINT_FILTERS = ["-readability/casting", "-runtime/int"]
 
@@ -106,11 +106,20 @@
         for filename, affected_lines in file_affected_line_map.iteritems():
             if filename.split(".")[-1] not in ("c", "h", "cc"):
                 continue
-            show_cmd = SHOW_CMD + [":" + filename]
-            show = Subprocess(show_cmd, stdout=subprocess.PIPE)
-            lint = Subprocess(cpplint_cmd, expected_returncode=(0, 1),
-                              stdin=show.stdout, stderr=subprocess.PIPE)
-            lint_out = lint.communicate()[1]
+
+            if args:
+                # File contents come from git
+                show_cmd = SHOW_CMD + [args[0] + ":" + filename]
+                show = Subprocess(show_cmd, stdout=subprocess.PIPE)
+                lint = Subprocess(cpplint_cmd, expected_returncode=(0, 1),
+                                  stdin=show.stdout, stderr=subprocess.PIPE)
+                lint_out = lint.communicate()[1]
+            else:
+                # File contents come from the working tree
+                lint = Subprocess(cpplint_cmd, expected_returncode=(0, 1),
+                                  stdin=subprocess.PIPE, stderr=subprocess.PIPE)
+                stdin = open(os.path.join(tl, filename)).read()
+                lint_out = lint.communicate(stdin)[1]
 
             for line in lint_out.split("\n"):
                 fields = line.split(":")