Option to merge tags in config files

[mentonin]

When backing up using config files, I would expect multiple tags declarations to be merged together. Instead, the first, most specific declaration is the only one considered. Some example cases:

# A.toml
[backup]
tags = [ "tag-A" ]

# B.toml
[backup]
tags = [ "tag-B" ]

# C.toml
[[backup.snapshots]]
tags = [ "tag-C" ]

• rustic backup -P A -P B results in tag-A.
• rustic backup -P A -P C results in tag-C.
• rustic backup -P A -P C --tag tag-D results in tag-D.
• rustic backup -P A -P C --tag tag-D --tag tag-E results in tag-D, tag-E.

I would expect every tag and --tag option to append to other tag declarations, both with multiple declarations and using use-profiles imports.

More specific use cases:

• I want to create a config file with many snapshot declarations, where every snapshot is tagged with a common tag, but some might have extra tags.
• I have a few modular config-files that I might apply on some backups, and want to tag those backups with each optional config used. For example, some combination of auto, manual, no-gitignore, no-backupignore, test, important

Overwriting tags may also be useful, of course. So maybe rustic would need to implement some kind of append-tags, exclude-tags, or something similar.