improve visualisation for state-transitions in unified diffs

[Byron]

In the unified diff view, we currently know the following states:

• added
• deleted
• renamed
• modified

In addition to this, the modification can change not only the content, but also the type of the entry.

• executable bit added
• executable bit removed
• symlink to blob
• blob to symlink
• other typechange
  • This is always involving submodules, i.e. submodule is replaced with a file, executable or link, or vice-versa
  • However, for submodule related changes

If I had to visualise this, I'd do as follows:

• have an icon for each of the types, i.e. executable, blob, link, submodule
• if there is a typechange, show some sort of a-icon -> b-icon so both icons are visible, instead of making up a description of the change in a pill. This is most flexible. However, it won't hurt to specifically have a pill for executable bit change, maybe, but in theory, it is now superfluous.

Follow-up of #10198 .

Tasks

• improve file-type icons to specifically handle additive executable bits (PR)
• transitions of file types (icon to icon)

[Byron]

This is just so that we don't forget after doing some work on this today, @PavelLaptev. Depending on how this is done, we'd only need some icons for executables, symlinks and submodules, along with a design for how the type changes should be visualised.
Also CC @estib-vega