Replace setuptools_git with a smarter approach

Implement a local egg_info command that only re-generates the
SOURCES.txt file when we need to. That is:

 - If there is no SOURCES.txt, make one
 - If we have run the sdist command, make one

Otherwise, leave well enough alone.

Also, skip doing any git processing if SKIP_GIT_SDIST is specified.

This should mean that consumers of our tarballs should not get screwed
by the need to inject git processing into the sdist.

Change-Id: I163b1c153d030e79b120600a2890edeb49e1fa90

pbr/util.py

@@ -77,6 +77,7 @@ try:
 except ImportError:
     import configparser
 
+from pbr import extra_files
 import pbr.hooks
 
 # A simplified RE for this; just checks that the line ends with version
@@ -258,23 +259,9 @@ def cfg_to_args(path='setup.cfg'):
         wrap_commands(kwargs)
 
         # Handle the [files]/extra_files option
-        extra_files = has_get_option(config, 'files', 'extra_files')
-        if extra_files:
-            extra_files = split_multiline(extra_files)
-            # Let's do a sanity check
-            for filename in extra_files:
-                if not os.path.exists(filename):
-                    raise DistutilsFileError(
-                        '%s from the extra_files option in setup.cfg does not '
-                        'exist' % filename)
-            # Unfortunately the only really sensible way to do this is to
-            # monkey-patch the manifest_maker class
-            @monkeypatch_method(manifest_maker)
-            def add_defaults(self, extra_files=extra_files, log=log):
-                log.info('[pbr] running patched manifest_maker command '
-                          'with extra_files support')
-                add_defaults._orig(self)
-                self.filelist.extend(extra_files)
+        files_extra_files = has_get_option(config, 'files', 'extra_files')
+        if files_extra_files:
+            extra_files.set_extra_files(split_multiline(files_extra_files))
 
     finally:
         # Perform cleanup if any paths were added to sys.path