Currently, merge commits can have a git tag, but they cannot have a
custom git commit ID.
This commit allows modifying the default merge commit id.
It also displays all merge commits IDs, which undoes
3ccf027f42
Previously, messages and notes that had multiple lines (via `<br>`-tags) were only displayed correctly
when using the default `center` value for the `messageAlign` and `noteAlign` configuration options.
Using `left` or `right` for the alignment options caused the text to collapse and become illegible,
as outlined in issue #3011.
This comes as a side-effect from how the internal `valign` text-rendering option was configured for
messages and notes:
```js
// Example from `sequenceRenderer.js: drawMessage()`
textObj.anchor = conf.messageAlign;
textObj.valign = conf.messageAlign;
```
Both the `anchor` option (which controls left-right alignment) and the `valign` option (which controls
vertical top-down placement) were set to the same value, the user-provided `messageAlign` config option.
While `left` and `right` are valid values for the `anchor` option, they were effectively no-ops for the
`valign` option, which only supports `top`, `start`, `middle`, `center`, `bottom`, and `end`.
To fix the issue, the `valign` property is now always set to `center` for notes and messages.
Similarly, the `dominantBaseline` option of texts is now always set to `middle`, rather than setting it to
either `text-{before,after}-edge`, which caused left-aligned multi-line text to always be "top-left" aligned
inside a note (or "bottom-right" when attempting to right-align).
Now, texts in messages and notes are always vertically centered and alignment options correctly apply for
both single and multi-line content.
