BUG: git clean -d cannot remove files from read-only directories

Repro steps:
1. git init 
2. mkdir d
3. touch d/a
4. chmod -w d
5. git clean -fd

Actual result: Git doesn't remove anything, saying "warning: failed to remove d/a".

Expected result: Git should remove the subdirectory 'd' along with its contents. Note that git can remove a read-only file (touch b; chmod -w b; git clean -f) with no problem.

Environment: git version 2.7.4 on GNU/Linux

Why this is important:
* This has a significant impact in real scenarios: the go language, when using modules, will download referenced modules and place them in read-only directories inside a cache directory, potentially inside the git repository. These cached modules can't be cleaned up by git clean. (Furthermore, git clean then returns a failure status code, which causes our build system to fail. I don't want to ignore the failure code.)

* There is no simple and reliable way to find out which directories I would need to chmod u+w in order to allow git to delete their contents. I'd have to effectively replicate git's algorithm for determining tracked files. "git clean" is invoked at the root of the repository and the repository contains many projects worked on by different teams, so there's no fixed list of things I can manually chmod to make "git clean" work reliably.

* It seems inconsistent for "git clean" to be able to remove read-only files but not files from read-only directories.