Run a configured command to prepare the copied tree before building

[sourcefrog]

For this specific issue (the .git thing), I found that editing SOURCE_EXCLUDE in the source and building my own binary was sufficient. In general, I think that some kind of "prepare the source tree" hook might be warranted for more complicated setups (e.g., something generates sources outside of build.rs).

Originally posted by @mathstuf in https://github.com/sourcefrog/cargo-mutants/issues/348#issuecomment-2445241475