Add plugin for dropping commits from output

plugins/drop/README.md

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+## Drop commits from output
+
+To use the plugin, add the command line flag `--plugin drop=<spec>`.
+The flag can be given multiple times to drop more than one commit.
+
+The <spec> value can be either
+
+ - a comma-separated list of hg hashes in the full form (40
+   hexadecimal characters) to drop the corresponding changesets, or
+
+ - a regular expression pattern to drop all changesets with matching
+   descriptions.

plugins/drop/__init__.py

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+from __future__ import print_function
+
+import sys, re
+
+
+def build_filter(args):
+    if re.match(r'([A-Fa-f0-9]{40}(,|$))+$', args):
+        return RevisionIdFilter(args.split(','))
+    else:
+        return DescriptionFilter(args)
+
+
+def log(fmt, *args):
+    print(fmt % args, file=sys.stderr)
+    sys.stderr.flush()
+
+
+class FilterBase(object):
+    def __init__(self):
+        self.remapped_parents = {}
+
+    def commit_message_filter(self, commit_data):
+        rev = commit_data['revision']
+
+        mapping = self.remapped_parents
+        parent_revs = [rp for p in commit_data['parents']
+                       for rp in mapping.get(p, [p])]
+
+        commit_data['parents'] = parent_revs
+
+        if self.should_drop_commit(commit_data):
+            log('Dropping revision %i.', rev)
+
+            self.remapped_parents[rev] = parent_revs
+
+            # Head commits cannot be dropped because they have no
+            # children, so detach them to a separate branch.
+            commit_data['branch'] = b'dropped-hg-head'
+            commit_data['parents'] = []
+
+    def should_drop_commit(self, commit_data):
+        return False
+
+
+class RevisionIdFilter(FilterBase):
+    def __init__(self, revision_hash_list):
+        super(RevisionIdFilter, self).__init__()
+        self.unwanted_hg_hashes = {h.encode('ascii', 'strict')
+                                   for h in revision_hash_list}
+
+    def should_drop_commit(self, commit_data):
+        return commit_data['hg_hash'] in self.unwanted_hg_hashes
+
+
+class DescriptionFilter(FilterBase):
+    def __init__(self, pattern):
+        super(DescriptionFilter, self).__init__()
+        self.pattern = re.compile(pattern.encode('ascii', 'strict'))
+
+    def should_drop_commit(self, commit_data):
+        return self.pattern.match(commit_data['desc'])