filter-repo: consolidate filtering functions into RepoFilter

Location of filtering logic was previously split in a confusing fashion
between FastExportFilter and RepoFilter.  Move all filtering logic from
FastExportFilter into RepoFilter, and rename the former to
FastExportParser to reflect this change.

One downside of this change is that FastExportParser's _parse_commit
holds two pieces of information (orig_parents and had_file_changes)
which are not part of the commit object but which are now needed by
RepoFilter.  Adding those bits of info to the commit object does not
make sense, so for now we pass an auxiliary dict with the
commit_callback that has these two fields.  This information is not
passed along to external commit_callbacks passed to RepoFilter, though,
which seems suboptimal.  To be fair, though, commit_callbacks to
RepoFilter never had access to this information so this is not a new
shortcoming, it just seems more apparent now.

Signed-off-by: Elijah Newren <newren@gmail.com>

t/t9391/unusual.py

@@ -21,7 +21,7 @@ import textwrap
 import git_filter_repo as fr
 
 total_objects = {'common': 0, 'uncommon': 0}
-def track_everything(obj):
+def track_everything(obj, *_ignored):
   if type(obj) == fr.Blob or type(obj) == fr.Commit:
     total_objects['common'] += 1
   else:
@@ -43,7 +43,7 @@ def handle_progress(progress):
 def handle_checkpoint(checkpoint_object):
   # Flip a coin; see if we want to pass the checkpoint through.
   if random.randint(0,1) == 0:
-    checkpoint_object.dump(filter._output)
+    checkpoint_object.dump(parser._output)
   track_everything(checkpoint_object)
 
 mystr = b'This is the contents of the blob'
@@ -53,25 +53,21 @@ compare = b"Blob:\n  blob\n  mark :1\n  data %d\n  %s" % (len(mystr), mystr)
 # upon.
 myblob = fr.Blob(mystr)
 assert bytes(myblob) == compare
-# Everyone should be using RepoFilter objects, not FastExportFilter.  But for
+# Everyone should be using RepoFilter objects, not FastExportParser.  But for
 # testing purposes...
-filter = fr.FastExportFilter('.',
-                             blob_callback   = track_everything,
+parser = fr.FastExportParser(blob_callback   = track_everything,
                              reset_callback  = track_everything,
                              commit_callback = track_everything,
                              tag_callback    = track_everything,
                              progress_callback = handle_progress,
                              checkpoint_callback = handle_checkpoint)
 
-filter.run(input = sys.stdin.detach(),
-           output = open(os.devnull, 'bw'),
-           fast_import_pipes = None,
-           quiet = True)
+parser.run(input = sys.stdin.detach(),
+           output = open(os.devnull, 'bw'))
 # DO NOT depend upon or use _IDS directly you external script writers.  I'm
 # only testing here for code coverage; the capacity exists to help debug
 # git-filter-repo itself, not for external folks to use.
 assert str(fr._IDS).startswith("Current count: 4")
-assert filter.num_commits_parsed() == 1
 print("Found {} blobs/commits and {} other objects"
       .format(total_objects['common'], total_objects['uncommon']))