Comments for commits

Comments on commits
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I will try to implement a way for creating comments in changesets. There are some parts of the project I'd like to explain and debate.

UI
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There will not be a lot of changes for the UI, and I will probably re-use parts of the existent UI:
### How to create a new comment for a changeset
The bubble icon for creating a comment on the changeset screen will be displayed in the second column of the file you want to comment on. I chose that column and not the first because the first one contains the line numbers of that file in the previous revision, and the purpose of this is to comment on the last revision.
### How to display a comment on the changeset
In the same way we are actually doing it for comments on files.

Javascript
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I will try to re-use as much code as possible, so I will try to modify the views for displaying comments (`CommentsForALineView`) and the comment bubble (`LineCommentBubblesView`) to be able to work on both commit and file comments, and not create new ones.

Storage
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The comments for commits will be stored in the same way the comments for files are. The required fields for a comment in commit register are a path (containing the filename), a line (containing the line of the revision we want to comment on) and a revision number.