fscache: compute correct symlink size in lstat()

In #2637, we fixed a bug
where symbolic links' target path sizes were recorded incorrectly in the
index.

However, we did so only in `mingw_lstat()` but not in `fscache_lstat()`.
Meaning: in code paths where the FSCache feature is enabled, Git _still_
got the wrong idea if the symbolic link target's length.

Let's fix this.

Note: as the FSCache feature reads in whole swaths of directory entries
in batch mode, even if metadata for only one of them might be required,
we save the expensive `CreateFile()` call that is required to compute
the symbolic link target's length to the `fscache_lstat()` call.

This fixes #2653.

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

compat/win32/fscache.c

@@ -597,6 +597,18 @@ int fscache_lstat(const char *filename, struct stat *st)
 	if (!fse)
 		return -1;
 
+	/*
+	 * Special case symbolic links: FindFirstFile()/FindNextFile() did not
+	 * provide us with the length of the target path.
+	 */
+	if (fse->u.s.st_size == MAX_LONG_PATH && S_ISLNK(fse->st_mode)) {
+		char buf[MAX_LONG_PATH];
+		int len = readlink(filename, buf, sizeof(buf) - 1);
+
+		if (len > 0)
+			fse->u.s.st_size = len;
+	}
+
 	/* copy stat data */
 	st->st_ino = 0;
 	st->st_gid = 0;