[12]Use the correct root for shared jail when the source storage is also a jail

[icewind1991]

Backport of #7137

[codecov]

Codecov Report

Merging #7323 into stable12 will increase coverage by 0.06%.
The diff coverage is 91.42%.

@@              Coverage Diff               @@
##             stable12    #7323      +/-   ##
==============================================
+ Coverage       53.86%   53.93%   +0.06%     
- Complexity      22596    22699     +103     
==============================================
  Files            1384     1384              
  Lines           86671    86953     +282     
  Branches         1329     1330       +1     
==============================================
+ Hits            46684    46894     +210     
- Misses          39987    40059      +72

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Files/Cache/Wrapper/CacheJail.php	88.37% <100%> (+0.27%)	39 <1> (+1)	⬆️
lib/private/Files/Storage/Wrapper/Jail.php	91.15% <71.42%> (+10.01%)	53 <2> (+2)	⬆️
apps/files_sharing/lib/Cache.php	89.23% <94.11%> (+2.19%)	25 <8> (+4)	⬆️
...ib/private/DB/QueryBuilder/CompositeExpression.php	50% <0%> (-12.5%)	6% <0%> (+3%)
lib/private/URLGenerator.php	67.01% <0%> (-4.05%)	64% <0%> (ø)
apps/files_external/lib/Command/ListCommand.php	37.76% <0%> (-2.91%)	37% <0%> (-2%)
lib/private/Group/MetaData.php	68.91% <0%> (-1.86%)	19% <0%> (ø)
lib/autoloader.php	79.62% <0%> (-1.63%)	27% <0%> (+12%)
lib/private/Diagnostics/Query.php	36.84% <0%> (-1.62%)	8% <0%> (+2%)
lib/private/Files/Cache/Propagator.php	94.93% <0%> (-1.27%)	16% <0%> (ø)
... and 39 more

[icewind1991]

Added #7385 to fix the performance regression

[rullzer]

Yes please

[MorrisJobke]

Tested and works 👍