Test that summer works fine when given a file instead of a directory.
$ alias summer="$TESTDIR/../summer"
$ touch empty
$ echo marola > hola
$ summer -v generate ./empty
"./empty": writing checksum \(checksum:0, mtime:\d+\) (re)
0s: 0 matched, 0 modified, 1 new, 0 corrupted
$ summer --parallel=1 -v verify .
"empty": match \(checksum:0, mtime:\d+\) (re)
"hola": missing checksum attribute
0s: 1 matched, 0 modified, 1 new, 0 corrupted
$ summer update ./hola
0s: 0 matched, 0 modified, 1 new, 0 corrupted
$ summer --parallel=1 -v verify .
"empty": match \(checksum:0, mtime:\d+\) (re)
"hola": match \(checksum:239059f6, mtime:\d+\) (re)
0s: 2 matched, 0 modified, 0 new, 0 corrupted