api: JSON omitempty on optional fields in FileMatch (#721)
There are several fields which are optional in FileMatch. For example
branch and version are only set if indexing a git repo (zoekt can index
arbitrary files). We mark these fields as optional to marshal in JSON.
The main benefit of this is improving the readability of some golden
files.
Note: I noticed this when an unrelated commit touched these golden
files.
Test Plan: go test