sign-my-commits fails when gpg-agent and pinentry-curses are being used

When trying to use sign-my-commits, with gpg-agent and pinentry-curses in use then the command fails with

$ bzr sign-my-commits
<email address hidden>

You need a passphrase to unlock the secret key for
user: "James Westby <email address hidden>"
1024-bit DSA key, ID B577FE13, created 2006-06-02

gpg: cancelled by user
gpg: no default secret key: bad passphrase
gpg: [stdin]: clearsign failed: bad passphrase
bzr: ERROR: Failed to gpg sign data with command '['gpg', '--clearsign']'

But works with pinentry-gtk2.

The problem can be seen that

cat test | gpg --clearsign fails, where gpg --clearsign test suceeds.

So

  process = subprocess.Popen(self._command_line(),
                                       stdin=subprocess.PIPE,
                                       stdout=subprocess.PIPE)

will cause it to fail in this case.

I think it is reasonable for pinentry-curses to work in this case, but maybe it could be handled more gracefully.

The solution I immediately thought of was to write the commit to a temporary file and pass that to gpg. But as LarstiQ pointed out this could be troublesome.

Hopfully someone smarter can think up a better solution.

James