This makes it such that annexed files are treated like plain git files
in comparisons (e.g. the diff of a commit).
It also changes the image diff viewer to show a more reasonable error
message when one of the annexed files under comparison is missing.
Fixes#56.
Reviewed-on: https://codeberg.org/forgejo-aneksajo/forgejo-aneksajo/pulls/57
Co-authored-by: Matthias Riße <m.risse@fz-juelich.de>
Co-committed-by: Matthias Riße <m.risse@fz-juelich.de>