gitk: Support showing the gathered inline diffs

gitk: Support showing the gathered inline diffs

The previous commit split the diffs into a separate field.  Now we
actually want to show them.

To that end we use the stored diff, and

- process it once to build a fake "tree diff", i.e., a list of all
  changed files;

- feed it through parseblobdiffline to actually format it into the
  $ctext field, like the existing diff machinery would.

Signed-off-by: Thomas Rast <redacted>
Signed-off-by: Paul Mackerras <redacted>