feat(snapshots): Callback for when uploader finishes processing a file (#2331)

* Make callback for upload file completion

Callback does not indicate that a file will be reachable immediately in
the resulting snapshot, but does indicate that the uploader is done
processing the file in some way (either via uploading data or finding a
previous version in the repo) and whether there was an error processing
the file.

* Tests for new FinishedFile callback

Ensure hadErr is properly populated and FinishedFile is called even if
the file was considered cached.

* Refine comment on interface function slightly

* Give callback error instead of bool about error

* Add locks around concurrent accesses in test

internal/mockfs/mockfs.go

@@ -115,6 +115,25 @@ func (imd *Directory) AddFile(name string, content []byte, permissions os.FileMo
 	return file
 }
 
+// AddFileWithSource adds a mock file with the specified name, permissions, and
+// given source function for getting a Reader instance.
+func (imd *Directory) AddFileWithSource(name string, permissions os.FileMode, source func() (ReaderSeekerCloser, error)) *File {
+	imd, name = imd.resolveSubdir(name)
+	file := &File{
+		entry: entry{
+			name:    name,
+			mode:    permissions,
+			size:    0,
+			modTime: DefaultModTime,
+		},
+		source: source,
+	}
+
+	imd.addChild(file)
+
+	return file
+}
+
 // AddSymlink adds a mock symlink with the specified name, target and permissions.
 func (imd *Directory) AddSymlink(name, target string, permissions os.FileMode) *Symlink {
 	imd, name = imd.resolveSubdir(name)