Merge lp://staging/~sinzui/bzr-gtk/per-file-commit into lp://staging/bzr-gtk

Ensure that the per/file message widget fits at least one line.

When I do bzr gcommit, a window is shown in which I can enter the per
file commit messages. But this per file commit window is too small. And
I am unable to make it big (and usable)

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RULES

    * This is a Gtk3 issue where the rules of expand and fill changed.
      The TextView was allocated 1 pixel which is not enough to see the
      text of the line.
    * This issue was partially addressed in a previously when the expand
      and fill rules were adjusted to solve other layout issues; the message
      has space for one line in trunk on precise at this movement.
    * There is no way to guarantee that space is allocated, but the widget
      could request the height of two lines when Gtk is allocating space.

    * ADDENDUM
      ./setup.py check -m commit had errors because DiffView could not
      be imported. I moved the inline import to a global import. If this
      should not happen, test_commit.py could import DiffView to ensure
      it is available for all the tests.

QA

    * Add
      per_file_commits = on
      to your bazaar or locations conf file.
    * Issue this command with this merged branch
      BZR_PLUGINS_AT=gtk@./ bzr gci
    * Select commit.py from the listed files.
    * Verify that "Commit message for commit.py" is expanded
      and that there is about 2 lines of space. (works for me)