[Backend] Add uniqueness constraint on SourceFileChange

[zergov]

A commit can only change a file in one way.

For example, It does not make sense to say:

• Commit "abcdef" changes file A by adding 2 lines
• Commit "abcdef" changes file A by adding 5 lines Instead, we say:
• Commit "abcdef" changes file A by adding 7 lines

This prevents recording multiple source file changes for the same file on a given commit.