siso: require a regular file in flushRegularFile's already-exist check

Before writing a file, flushRegularFile checks whether the correct
file is already on disk. matchesFileInfo compared only size and
mtime, so a stale symlink could pass the check by accident: a
symlink's lstat size is the length of its target string, and on
filesystems with coarse mtimes the timestamps can collide too. Flush
then left a symlink at a path where a regular file belongs. The TODO
in the code already pointed at the missing mode check.

Require fi.Mode().IsRegular(). A non-regular entry now falls through
to flushRemoveReason, which removes it before the write.

Change-Id: Ia575ce1e2b58036a79fdfcfa498bfce6c1d968fb
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8037825
Reviewed-by: Yuke Liao <liaoyuke@google.com>
Auto-Submit: Philipp Wollermann <philwo@google.com>
Commit-Queue: Philipp Wollermann <philwo@google.com>