backup --files-from-verbatim allow merging snapshots

[gmgigi96]

What does this PR change? What problem does it solve?

Until now, when --files-from-verbatim option is used in a backup, restic creates a new snapshot containing only the files specified in the file passed in that option. But when a repository is used to backup files of the same directory, it would be nice have all the snapshots with a full view of the folder, while specifying only the updated (new, modified and removed) files or directories during the backup process.

This PR adds an new option into backup, --merge, that will merge the parent snapshot with the current list of files specified in --files-from-verbatim.
This has two pros:

• backuping folders with millions of files doesn't led to a full scan of them, wasting a lot of time. With a list of updated files, restic has to look after only to those files.
• the restore process is the same as a snapshot created with a "full backup", with the possibility to fully restore the folder from a single snapshot.

Example:

> exa -T dir
dir
└── other
   ├── other-file1
   └── other-file2

> export RESTIC_PASSWORD=password

> restic -r repo init
[...]

> restic -r repo backup dir/
repository 7d31e4a2 opened successfully, password is correct
created new cache in /home/gmgigi/.cache/restic
no parent snapshot found, will read all files

Files:           2 new,     0 changed,     0 unmodified
Dirs:            2 new,     0 changed,     0 unmodified
Added to the repo: 1.344 KiB

processed 2 files, 0 B in 0:00
snapshot 9adcd3fc saved

> (creating a new folder with some files inside)

> exa -T dir
dir
├── other
│  ├── other-file1
│  └── other-file2
└── subdir
   ├── bar
   └── foo

> cat list
dir/subdir/foo
dir/subdir/bar

[list contains only the new files, but can contains also deleted and updated files, see the unit tests for more examples]

> restic -r repo backup --files-from-verbatim list --parent 9adcd3fc --merge
repository 7d31e4a2 opened successfully, password is correct
using parent snapshot 9adcd3fc

Files:           2 new,     0 changed,     0 unmodified
Dirs:            1 new,     1 changed,     0 unmodified
Added to the repo: 1.690 KiB

processed 2 files, 0 B in 0:00
snapshot 950708ff saved

> restic -r repo  snapshots
repository 7d31e4a2 opened successfully, password is correct
ID        Time                 Host        Tags        Paths
--------------------------------------------------------------------------
9adcd3fc  2021-06-01 09:46:23  avatar                  /home/gmgigi/dir
950708ff  2021-06-01 09:48:34  avatar                  /home/gmgigi/dir
--------------------------------------------------------------------------
2 snapshots

[mount restic repo]

> exa -T mount/snapshots
mount/snapshots
├── 2021-06-01T09:46:23+02:00
│  └── dir
│     └── other
│        ├── other-file1
│        └── other-file2
├── 2021-06-01T09:48:34+02:00
│  └── dir
│     ├── other
│     │  ├── other-file1
│     │  └── other-file2
│     └── subdir
│        ├── bar
│        └── foo
└── latest -> 2021-06-01T09:48:34+02:00

Was the change discussed in an issue or in the forum before?

https://forum.restic.net/t/backup-parent-behavior/3286

Closes #3118

Checklist

• I have read the Contribution Guidelines
• I have enabled maintainer edits for this PR
• I have added tests for all changes in this PR
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review

[robvalca]

Awesome work. For our use case, we can guess the files to backup before calling restic, and this feature will reduce drastically the time to backup ! Looking forward to see this merged!

[gmgigi96]

Resolved the conflicts with the master.

[gmgigi96]

Changed the library used in the tests for writing into files, in order to be compatible with previous go versions

[gmgigi96]

@fd0 @MichaelEischer Any feedback for this? :)