Empty files can be confusing in diff output. For example:
```
Added regular file file1:
Added regular file file2:
1: foo
```
This commit adds an "(empty)" placeholder instead. Since it's not
colored, and doesn't have line numbers, it will hopefully not be
mistaken for a file with the contents "(empty)".
AFAIK, we can't make HEAD detached in an empty Git repository, so we need
to temporarily switch to the new default branch before checking out.
Fixes#2047
They are shown next to the change and commit id, since they are other names the
commit can be referred by.
The description is separated from the branches by a ` | ` when there are
branches, so that one can tell the branches from the description without color.
The result looks like this: 
Maybe we could load GitBackend without resolving .git symlink, but that would
introduce more subtle bugs. Instead, we calculate the expected Git workdir path
from the canonical ".git" path.
Fixes#2011
