This patch allows to obtain snapshots' file delta (aka Snap Diff) by
reading fake 'snapdiff-query-formatted' subfolders under .snap
directory.
Snapdiff subfolders are not visible when reading from .snap folder, one
has to build and issue such a "query" manually.

Resulting output (directory listing) contains just entries which have
been altered (created/updated/removed) in the final shapshot since the
initial one. New/updated entries are presented as regular files, names
of the removed ones are prefixed with tilda '~'.
E.g. to compare snapshots named snap1 and snap2 one can issue:
>ls -l /mnt/mycephfs/dir0/.snap/.~diff=snap1.~diff=snap2

which would return something like that:
total 8
-rw-r--r-- 1 root root  3 Jul 19 16:40 b
-rw-r--r-- 1 root root  3 Jul 19 16:40 ~c
drwxr-xr-x 0 root root  0 Jul 19 16:40 ~C
-rw-r--r-- 1 root root  3 Jul 19 16:40 d
-rw-r--r-- 1 root root  3 Jul 19 16:40 f
-rw-r--r-- 1 root root  3 Jul 19 16:40 ~g
drwxr-xr-x 0 root root  0 Jul 19 16:40 ~G
drwxr-xr-x 0 root root  0 Jul 19 16:40 I
-rw-r--r-- 1 root root  3 Jul 19 16:40 k
drwxr-xr-x 0 root root  0 Jul 19 16:40 K
-rw-r--r-- 1 root root  3 Jul 19 16:40 l
drwxr-xr-x 4 root root 12 Jul 19 16:41 L
drwxr-xr-x 2 root root  6 Jul 19 16:40 S
drwxr-xr-x 2 root root  3 Jul 19 16:40 T

or
> ls -l /mnt/mycephfs/dir0/.snap/.~diff=snap1.~diff=snap3
total 7.5K
-rw-r--r-- 1 root root  3 Jul 19 16:40 a
-rw-r--r-- 1 root root  3 Jul 19 16:40 b
-rw-r--r-- 1 root root  3 Jul 19 16:40 ~c
drwxr-xr-x 0 root root  0 Jul 19 16:40 ~C
-rw-r--r-- 1 root root  3 Jul 19 16:40 d
-rw-r--r-- 1 root root  3 Jul 19 16:40 ~f
-rw-r--r-- 1 root root  3 Jul 19 16:40 g
drwxr-xr-x 0 root root  0 Jul 19 16:40 ~G
drwxr-xr-x 0 root root  0 Jul 19 16:41 G
drwxr-xr-x 2 root root  3 Jul 19 16:40 H
drwxr-xr-x 0 root root  0 Jul 19 16:40 I
-rw-r--r-- 1 root root  3 Jul 19 16:40 l
drwxr-xr-x 4 root root 12 Jul 19 16:41 L
drwxr-xr-x 2 root root  6 Jul 19 16:40 S
drwxr-xr-x 2 root root  3 Jul 19 16:40 T

then diving deeper in the subfolder might show:
> ls -l /mnt/mycephfs/dir0/.snap/.~diff=snap1.~diff=snap2/~C
total 1
drwxr-xr-x 0 root root 0 Jul 19 16:40 ~C1
-rw-r--r-- 1 root root 3 Jul 19 16:40 ~cc1

and so on and so forth:
> ls -l /mnt/mycephfs/dir0/.snap/.~diff=snap1.~diff=snap2/~C/~C1
total 1
-rw-r--r-- 1 root root 6 Jul 19 16:40 ~c2

File content reading is also available. It returns the full(!) file
content in the target snapshot for new/updated files and one in the
initial snapshot for removed files.
E.g.
> less /mnt/mycephfs/dir0/.snap/.~diff=snap1.~diff=snap2/~C/~C1/~c2
snap1

Order of snapshot names in a snapdiff ""query" isn't important - they're
properly sorted properly according to their ids when processed.
Comparing snapshot and live data isn't supported. Byte-level "deltas"
are not supported.

Signed-off-by: Denis Barahtanov <denis.barahtanov@croit.io>