$ cat >findbranch.py <<EOF

  > import re, sys

  > 

  > head_re = re.compile('^#(?:(?:\\s+([A-Za-z][A-Za-z0-9_]*)(?:\\s.*)?)|(?:\\s*))$')

  > 

  > for line in sys.stdin:

  >     hmatch = head_re.match(line)

  >     if not hmatch:

  >         sys.exit(1)

  >     if hmatch.group(1) == 'Branch':

  >         sys.exit(0)

  > sys.exit(1)

  > EOF

  $ hg init a

  $ cd a

  $ echo "Rev 1" >rev

  $ hg add rev

  $ hg commit -m "No branch."

  $ hg branch abranch

  marked working directory as branch abranch

  $ echo "Rev  2" >rev

  $ hg commit -m "With branch."

  >     echo "Export of default branch revision has Branch header" 1>&2

  >     exit 1

  > fi

  >     :  # Do nothing

  > else

  >     echo "Export of branch revision is missing Branch header" 1>&2

  >     exit 1

  > fi

Make sure import still works with branch information in patches.

  $ hg init b

  $ cd b

  $ cd ..