Add head2branch plugin

Support converting unnamed heads to named branches during mercurial
conversions.

Co-Authored-By:	ostan89@gmail.com

README.md

@@ -201,6 +201,8 @@ exactly one head each. Otherwise commits to the tip of these heads
 within the branch will get flattened into merge commits. Chris J
 Billington's [hg-export-tool] can help you to handle branches with
 duplicate heads.
+Alternatively, you can use the [head2branch plugin](./plugins/head2branch)
+to create a new named branch from an unnamed head.
 
 hg-fast-export will ignore any files or directories tracked by mercurial
 called `.git`, and will print a warning if it encounters one. Git cannot

plugins/head2branch/README.md

@@ -0,0 +1,12 @@
+## Convert Head to Branch
+
+`fast-export` can only handle one head per branch. This plugin makes it possible
+to create a new branch from a head by specifying the new branch name and
+the first divergent commit for that head.  The revision number for the commit
+should be in decimal form.
+
+Note: you must run `fast-export` with `--ignore-unnamed-heads` option,
+otherwise, the conversion will fail.
+
+To use the plugin, add the command line flag `--plugin head2branch=name,<rev>`.
+The flag can be given multiple times to name more than one head.

plugins/head2branch/__init__.py

@@ -0,0 +1,23 @@
+import sys
+
+def build_filter(args):
+    return Filter(args)
+
+class Filter:
+
+    def __init__(self, args):
+        args = args.split(',')
+        self.branch_name = args[0]
+        self.starting_commit = int(args[1])
+        self.branch_parents = set()
+
+    def commit_message_filter(self, commit_data):
+        rev = commit_data['revision']
+        rev_parents = commit_data['parents']
+        if (rev == self.starting_commit
+            or any(rp in self.branch_parents for rp in rev_parents)
+            ):
+            self.branch_parents.add(rev)
+            commit_data['branch'] = self.branch_name.encode('ascii', 'replace')
+            sys.stderr.write('\nchanging r%s to branch %r\n' % (rev, self.branch_name))
+            sys.stderr.flush()