Add git.DIFF_RENAME_SIMILARITY_THRESHOLD option

[silverwind]

This makes the threshold value passed to git diff --find-renames configurable. Git's default is 50% and this sets the same value. For example, here is a file rename and change with 3 out of 4 changed lines with a value of 25%:

The reason for this change is the rewrite+rename in #36118 and I checked the similarity index there, and it's only 21%, so will need a value of 20% or less:

diff --git a/modules/storage/s3.go b/modules/storage/minio.go
similarity index 21%

[silverwind]

Maybe 25% is too prone to false-positive detections. Maybe I will revert to 50%, which is a safe default and I see why git has chosen it.

[silverwind]

Reverted to 50%. Ideally I think this could be a UI option, but it being a server option will suffice initially.

[TheFox0x7]

Not sure if that's a concern given the existing issues with settings but shouldn't settings value be checked? Otherwise git will error out later at runtime

[silverwind]

Hmm I guess we can add a small validation like ^([0-9]|[1-9][0-9]|100)%$ to allow only integers between 0 and 100.

[silverwind]

Validation added.

[silverwind]

PS: I also checked git diff config and it appears there is no config option to alter the 50% default, so I assume it can only be passed via CLI, so having this option in gitea is valuable.

[silverwind]

Docs: https://gitea.com/gitea/docs/pulls/316