Resolve name collisions

At the moment when two items to be saved have the same directory name,
restic only saves the first one to the repo. Let's say we have a
structure like this:

    dir1
    └── subdir
        └── file
    dir2
    └── subdir
        └── file

When restic is run on `dir1/subdir` and `dir2/subdir`, it will only save
the first `subdir`:

    $ restic backup dir1/subdir dir2/subdir
    [...]

    $ restic ls -l latest
    drwxr-xr-x  1000   100      0 2017-08-27 20:56:39 /subdir
    -rw-r--r--  1000   100     17 2017-08-27 20:56:39 /subdir/file

That's obviously a bad thing, caused by an early decision to strip the
full path to the files/dirs to save and only leave the last directory.

This commit partly resolves this by handling colliding names and
resolving the conflicts. Restic will now append a counter to the file
(`-123`) until the conflict is resolved. So in the example above, we'll
end up with the following structure:

    $ restic ls -l latest
    drwxr-xr-x  1000   100      0 2017-08-27 20:56:39 /subdir
    -rw-r--r--  1000   100     17 2017-08-27 20:56:39 /subdir/file
    drwxr-xr-x  1000   100      0 2017-08-27 20:56:46 /subdir-1
    -rw-r--r--  1000   100     17 2017-08-27 20:56:46 /subdir-1/file

This partly addresses #549 and closes #1179.

At first I thought that the obvious correction would be to archive the
full path. But it turns out that collisions may still occur: Suppose you
have a file named `foo` in the current directory, and the parent directory
also contains a file `foo`. Archiving these with restic also causes a
collision, since restic strips the `../` from the first file:

    $ restic backup ../foo foo

This also happens with `tar`, which does not handle the collision and
will happily archive two files called `foo`.

So, the best way forward is to handle name collisions and archive the
whole path. The latter will be tackled in a separate PR.

internal/archiver/archiver.go

@@ -383,7 +383,22 @@ func (arch *Archiver) dirWorker(ctx context.Context, wg *sync.WaitGroup, p *rest
 						panic("invalid null subtree restic.ID")
 					}
 				}
-				tree.Insert(node)
+
+				// insert node into tree, resolve name collisions
+				name := node.Name
+				i := 0
+				for {
+					i++
+					err := tree.Insert(node)
+					if err == nil {
+						break
+					}
+
+					newName := fmt.Sprintf("%v-%d", name, i)
+					fmt.Fprintf(os.Stderr, "%v: name collision for %q, renaming to %q\n", filepath.Dir(node.Path), node.Name, newName)
+					node.Name = newName
+				}
+
 			}
 
 			node := &restic.Node{}