Resolve file symlinks in git tree
When traversing the git tree in the content aggregator, Antora should locate and resolve file symlinks. Currently, it skips these entries.
By default, the tree walk facility in nodegit skips entries which are symlinks. In order to locate them, we must disable the default blob filter and check the filemode ourselves. We assume that if we find a symlink in the git tree, then it points to a file.
UPDATE: We have learned from the initial prototype that we will be able to support both file and directory symlinks at any depth.
The filemode that nodegit uses for symlinks confuses vinyl into writing a file w/ no permissions. Therefore, we'll also need to set the filemode on the virtual file to match that of a normal file (TreeEntry.FILEMODE.BLOB). (we are now using isomorphic-git as the git client, so this statement is no longer relevant).