Incremental mode doesn't work with group_albums

On `master:HEAD`, aborting an import (even with the incremental option set to `yes` in config) seems to disregard any progress, as the next time I run `import` it starts from the beginning.

``` sh
⇨ beet config

directory: ~/Dropbox/music/collection

import:
write: yes
copy: yes
move: no
incremental: yes
group_albums: yes
log: ~/Dropbox/music/beets/beets.log
threaded: yes

ui:
color: yes
library: ~/Dropbox/music/beets/library.db

⇨ beet import ~/Music/collection

None (1 items)
Finding tags for album "MF DOOM - MM..FOOD".
Candidates:
1. MF DOOM - MM..FOOD (58.1%) (missing tracks) (CD, 2007, US, Rhymesayers Entertainment)
2. MF DOOM - MM..FOOD (40.2%) (missing tracks, id, year, ...) (CD, 2004, US, Rhymesayers Entertainment)
3. MF DOOM - MM..More Food (35.5%) (missing tracks, id, tracks, ...) (CD, 2004, US)
4. MF DOOM - MM..Leftovers (29.7%) (missing tracks, id, album, ...) (CD, 2004, US)
5. MF DOOM - MM..FOOD (23.8%) (missing tracks, id, tracks, ...) (2004)
# selection (default 1), Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort?
Tagging:
MF DOOM - MM..FOOD
URL:
http://musicbrainz.org/release/2e57e853-97c4-3137-9206-8b712671a4d6
(Similarity: 58.1%) (missing tracks) (CD, 2007, US, Rhymesayers Entertainment)
Missing tracks (14/15 - 93.3%):
! Beef Rapp (#1) (4:39)
! Hoe Cakes (#2) (3:54)
! Potholderz (#3) (3:20)
! One Beer (#4) (4:18)
! Deep Fried Frenz (#5) (4:59)
! Poo-Putt Platter (#6) (1:13)
! Fillet-O-Rapper (#7) (1:03)
! Gumbo (#8) (0:49)
! Fig Leaf Bi-Carbonate (#9) (3:19)
! Kon Karne (#10) (2:51)
! Kon Queso (#12) (4:00)
! Rapp Snitch Knishes (#13) (2:52)
! Vomitspit (#14) (2:48)
! Kookies (#15) (4:01)
Apply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort? A
This album is already in the library!
Old: 1 items, MP3, 192kbps, 4:41, 6.5 MB
New: 1 items, MP3, 192kbps, 4:41, 6.5 MB
[S]kip new, Keep both, Remove old? R
None (1 items)
Finding tags for album "6th Borough Project - One Night in the Borough".
Candidates:
1. 6th Borough Project - One Night in the Borough (58.1%) (missing tracks) (Digital Media, 2011, XW, Delusions of Grandeur)
2. 6th Borough Project - One Night In The Borough (Part Two) (47.5%) (id, missing tracks, tracks, ...) (Digital Media, Delusions of Grandeur)
3. 6th Borough Project - One Night in the Borough (41.2%) (missing tracks, id, catalognum) (Digital Media, 2011, XW, Delusions of Grandeur)
4. 6th Borough Project - One Night in the Borough (40.5%) (missing tracks, id) (Digital Media, 2015, XW, Delusions of Grandeur, iTunes release)
5. 6th Borough Project - One Night in the Borough (37.8%) (missing tracks, id, media, ...) (CD, 2011, Delusions of Grandeur)
# selection (default 1), Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort? b

⇨ beet import ~/Music/collection

None (1 items)
Finding tags for album "MF DOOM - MM..FOOD".
Candidates:
1. MF DOOM - MM..FOOD (58.1%) (missing tracks) (CD, 2007, US, Rhymesayers Entertainment)
2. MF DOOM - MM..FOOD (40.2%) (missing tracks, id, year, ...) (CD, 2004, US, Rhymesayers Entertainment)
3. MF DOOM - MM..More Food (35.5%) (missing tracks, id, tracks, ...) (CD, 2004, US)
4. MF DOOM - MM..Leftovers (29.7%) (missing tracks, id, album, ...) (CD, 2004, US)
5. MF DOOM - MM..FOOD (23.8%) (missing tracks, id, tracks, ...) (2004)
# selection (default 1), Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort?
```

Start with the `beet import` command and the `incremental` and `group_albums` configuration paths shown in the report. Reproduce the abort-and-restart sequence, then verify that a subsequent import resumes past the already processed album rather than starting over.

Written by the indexing model from the issue text.