exclude files untracked by git from flake8 check

@krischer, maybe can you check if we need something like in #492?

obspy/core/tests/test_code_formatting.py

@@ -4,6 +4,7 @@
 import inspect
 import os
 import unittest
+from obspy.core.util.misc import get_untracked_files_from_git
 
 EXCLUDE_FILES = [
     "*/__init__.py",
@@ -34,13 +35,17 @@ def test_flake8(self):
         obspy_dir = os.path.dirname(os.path.dirname(os.path.dirname(test_dir)))
         error_count = 0
         file_count = 0
+        untracked_files = get_untracked_files_from_git() or []
         for dirpath, _, filenames in os.walk(obspy_dir):
             filenames = [_i for _i in filenames if
                          os.path.splitext(_i)[-1] == os.path.extsep + "py"]
             if not filenames:
                 continue
             for py_file in filenames:
                 py_file = os.path.join(dirpath, py_file)
+                # ignore untracked files
+                if os.path.abspath(py_file) in untracked_files:
+                    continue
 
                 # Check if the filename should not be excluded.
                 skip_file = False

obspy/core/util/misc.py

@@ -8,6 +8,9 @@
     GNU Lesser General Public License, Version 3
     (http://www.gnu.org/copyleft/lesser.html)
 """
+import os
+import inspect
+from subprocess import Popen, PIPE
 import warnings
 import itertools
 import numpy as np
@@ -217,6 +220,29 @@ def loadtxt(*args, **kwargs):
         return np.atleast_1d(data)
 
 
+def get_untracked_files_from_git():
+    """
+    Tries to return a list of files (absolute paths) that are untracked by git
+    in the repository.
+
+    Returns `None` if the system call to git fails.
+    """
+    dir_ = os.path.abspath(
+        os.path.dirname(inspect.getfile(inspect.currentframe())))
+    dir_ = os.path.dirname(os.path.dirname(os.path.dirname(dir_)))
+    try:
+        p = Popen(['git', 'status', '-u', '--porcelain'],
+                  cwd=dir_, stdout=PIPE, stderr=PIPE)
+        p.stderr.close()
+        stdout = p.stdout.readlines()
+        files = [os.path.abspath(os.path.join(dir_, line.split()[1].strip()))
+                 for line in stdout
+                 if line.startswith("??")]
+    except:
+        return None
+    return files
+
+
 if __name__ == '__main__':
     import doctest
     doctest.testmod(exclude_empty=True)