Merge pull request #3791: Various fixes around safe.directory

The first three commits are rebased versions of those in gitgitgadget#1215. These allow the following:

1. Fix `git config --global foo.bar <path>` from allowing the `<path>`. As a bonus, users with a config value starting with `/` will not get a warning about "old-style" paths needing a "`%(prefix)/`".

2. When in WSL, the path starts with `/` so it needs to be interpolated properly. Update the warning to include `%(prefix)/` to get the right value for WSL users. (This is specifically for using Git for Windows from Git Bash, but in a WSL directory.)

3. When using WSL, the ownership check fails and reports an error message. This is noisy, and happens even if the user has marked the path with `safe.directory`. Remove that error message.

compat/mingw.c

@@ -3107,9 +3107,7 @@ int is_path_owned_by_current_sid(const char *path, struct strbuf *report)
 				    DACL_SECURITY_INFORMATION,
 				    &sid, NULL, NULL, NULL, &descriptor);
 
-	if (err != ERROR_SUCCESS)
-		error(_("failed to get owner for '%s' (%ld)"), path, err);
-	else if (sid && IsValidSid(sid)) {
+	if (err == ERROR_SUCCESS && sid && IsValidSid(sid)) {
 		/* Now, verify that the SID matches the current user's */
 		static PSID current_user_sid;
 		BOOL is_member;

setup.c

@@ -1777,10 +1777,19 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		break;
 	case GIT_DIR_INVALID_OWNERSHIP:
 		if (!nongit_ok) {
+			struct strbuf prequoted = STRBUF_INIT;
 			struct strbuf quoted = STRBUF_INIT;
 
 			strbuf_complete(&report, '\n');
-			sq_quote_buf_pretty(&quoted, dir.buf);
+
+#ifdef __MINGW32__
+			if (dir.buf[0] == '/')
+				strbuf_addstr(&prequoted, "%(prefix)/");
+#endif
+
+			strbuf_add(&prequoted, dir.buf, dir.len);
+			sq_quote_buf_pretty(&quoted, prequoted.buf);
+
 			die(_("detected dubious ownership in repository at '%s'\n"
 			      "%s"
 			      "To add an exception for this directory, call:\n"