o/registrystate: support snapctl get --pristine

[MiguelPires]

Support snapctl get --pristine ... which allows snaps to read the state of the view disregarding the pending changes in the current transaction. Includes a small refactor to the registrystate functions that was necessary.

[codecov]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.87%. Comparing base (ac897ee) to head (a4bab62).
Report is 44 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/hookstate/ctlcmd/get.go	72.72%	2 Missing and 1 partial ⚠️
overlord/registrystate/registrystate.go	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14552      +/-   ##
==========================================
+ Coverage   78.85%   78.87%   +0.01%     
==========================================
  Files        1079     1082       +3     
  Lines      145615   146054     +439     
==========================================
+ Hits       114828   115200     +372     
- Misses      23601    23655      +54     
- Partials     7186     7199      +13     

Flag	Coverage Δ
unittests	78.87% <82.14%> (+0.01%)	⬆️

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[ZeyadYasser]

Changes look really good, Thank you!

I am curious about the intended use case for a snap to use --pristine? can't a snap just get a copy of the view data before adding changes to the transaction and get the same data?

It would be great to have comment that explains why we have it.

[MiguelPires]

I am curious about the intended use case for a snap to use --pristine? can't a snap just get a copy of the view data before adding changes to the transaction and get the same data?

A snap might be deciding whether to allow changes made by another snap, in which case it might be useful to know what was the previous state and there would be no other way of getting this data. Even if it's the same snap, several hooks are called in different stages so the snap would have to retain that data in the snap and access it across different hooks