The issue is duplicated code. There are two functions that zero-fill
the frame number. They worked the same for positive frames numbers, but
behaved differently for negative ones.
On frame `-100`, `BLI_path_frame` outputs `-0100` and
`fluid_cache_get_framenr_formatted_$ID$` outputted `-100`.
I changed the behavior of the latter, because we depend on the behavior
of the former for much longer already.
Reviewers: sebbas
Differential Revision: https://developer.blender.org/D8107
1e255ce031