clean: remove mount points when possible

Windows' equivalent to "bind mounts", NTFS junction points, can be
unlinked without affecting the mount target. This is clearly what users
expect to happen when they call `git clean -dfx` in a worktree that
contains NTFS junction points: the junction should be removed, and the
target directory of said junction should be left alone (unless it is
inside the worktree).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/clean.c

@@ -34,8 +34,10 @@ static const char *msg_remove = N_("Removing %s\n");
 static const char *msg_would_remove = N_("Would remove %s\n");
 static const char *msg_skip_git_dir = N_("Skipping repository %s\n");
 static const char *msg_would_skip_git_dir = N_("Would skip repository %s\n");
+#ifndef CAN_UNLINK_MOUNT_POINTS
 static const char *msg_skip_mount_point = N_("Skipping mount point %s\n");
 static const char *msg_would_skip_mount_point = N_("Would skip mount point %s\n");
+#endif
 static const char *msg_warn_remove_failed = N_("failed to remove %s");
 static const char *msg_warn_lstat_failed = N_("could not lstat %s\n");
 static const char *msg_skip_cwd = N_("Refusing to remove current working directory\n");
@@ -178,13 +180,24 @@ static int remove_dirs(struct strbuf *path, const char *prefix, int force_flag,
 	}
 
 	if (is_mount_point(path)) {
+#ifndef CAN_UNLINK_MOUNT_POINTS
 		if (!quiet) {
 			quote_path(path->buf, prefix, &quoted, 0);
 			printf(dry_run ?
 			       _(msg_would_skip_mount_point) :
 			       _(msg_skip_mount_point), quoted.buf);
 		}
 		*dir_gone = 0;
+#else
+		if (!dry_run && unlink(path->buf)) {
+			int saved_errno = errno;
+			quote_path(path->buf, prefix, &quoted, 0);
+			errno = saved_errno;
+			warning_errno(_(msg_warn_remove_failed), quoted.buf);
+			*dir_gone = 0;
+			ret = -1;
+		}
+#endif
 
 		goto out;
 	}

compat/mingw.h

@@ -452,6 +452,7 @@ static inline void convert_slashes(char *path)
 struct strbuf;
 int mingw_is_mount_point(struct strbuf *path);
 #define is_mount_point mingw_is_mount_point
+#define CAN_UNLINK_MOUNT_POINTS 1
 #define PATH_SEP ';'
 char *mingw_query_user_email(void);
 #define query_user_email mingw_query_user_email

t/t7300-clean.sh

@@ -794,6 +794,7 @@ test_expect_success MINGW 'clean does not traverse mount points' '
 	git init with-mountpoint &&
 	cmd //c "mklink /j with-mountpoint\\mountpoint target" &&
 	git -C with-mountpoint clean -dfx &&
+	test_path_is_missing with-mountpoint/mountpoint &&
 	test_path_is_file target/dont-clean-me
 '