fix!: Don't panic when reading empty files

[aawsome]

There was a bug when using partition_point which in that case does not gave guarentees about its behavior. Hence the whole logic was refactored and unit/integration tests were added.

closes rustic-rs/rustic#1380

[codecov]

Codecov Report

Attention: Patch coverage is 67.74194% with 10 lines in your changes missing coverage. Please review.

Project coverage is 42.2%. Comparing base (bf6c7d0) to head (28e7968).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/vfs.rs	66.6%	10 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/repository.rs	42.2% <100.0%> (ø)
crates/core/tests/integration.rs	84.6% <ø> (+3.8%)	⬆️
crates/core/src/vfs.rs	16.9% <66.6%> (+6.1%)	⬆️

... and 18 files with indirect coverage changes