Add visual diff

[davelab6]

Following #246,

• Change the deploy script to not delete the previous built, but deploy each subdirectory of /builds/ (that is, all the $gitHash.buildID/ directories) to the gh-pages branch
• Change the AngularJS browser to enable comparing builds.

First thing is a timeline browser. Simplest thing would be a paginated table with a list of the builds, like in the old FontBakery Web App, that allows you to select 2 builds and hit a compare button.

(In the future this will become a long list, so filtering will be needed. To limit the table to a smaller time span, a graph like a Github year-long block graph of activity could work well to visualise intensity of activity:

If this could be 'scaled' up to decade and down to month, week, day, you could pick an interesting way point easily. But that's for far in the future.)

• There should be a toggle/checkbox to show/hide unchanged files
• First comparison is of directory tree structure.
  The addition/removal of files could be done with the existing JSON-Diff JS that is used to compare METADATA.json files, if a python os.walk() is used to output a JSON representation of the directory that is deployed... However, we also need to show filesize changes, so perhaps it could be done with a 2 pane (or 2+ pane) view like http://meldmerge.org uses.
• Second comparison is what files changed. For glyphs, we can use Computer Vision to detect when a glyph changes, and by how much, and then show a table of the glyphs that have changed, like http://nodebox.github.io/opentype.js/glyph-inspector.html , using each of the different comparison modes shown in http://xuv.github.io/design-with-git/
• When you select a text file from the first comparison mode, we can show a regular diff.
• When you select a YAML file from the first comparison mode, we can show a YAML diff like we do for METADATA.json
• When you select a XML file from the first comparison mode, we can show a XML diff

(Again, the old Font Bakery Web App did the YAML/XML diff and might have code we can reuse)

[davelab6]

@andriyko do you think you may complete this before Dec 1? :)

[davelab6]

Okay, no time for this year :)

[davelab6]

Similar discussion at googlefonts/roboto-2#30

[davelab6]

google/fonts#24 fixed an issue that this should be catching

here are the broken originals:

Here are the fixed fonts:

@graphicore has developed a project specific comparison tool which could be the basis for something PhantomJS based here: http://tarobish.github.io/Jomhuria/#tests/collision-above-3

[davelab6]

If winniethemu/tiff#2 is closed, potentially embed tiff :) There are now more font diffing tools; https://typeresources.github.io/glyph-gazer/ and https://github.com/googlei18n/fontdiff and perhaps more.