contenthash: unconditionally clear ModeIrregular during content hash

Go 1.23 changed the behavior of `os.Stat` to set `ModeIrregular` when it
encountered a mount point reparse point on Windows. These were
previously labeled as symlinks.

We attempted to fix this by clearing `ModeIrregular` during checksum
generation, but we only cleared it in the case where `ModeIrregular` and
at least one other mode type was set.

After reviewing the original solution and issue, I believe that this was
too conservative. The original issue seems to indicate that only the
`ModeIrregular` bit was set so the condition would not have triggered
and cleared it properly. I thought mount points would have both
`ModeDir` and `ModeIrregular` set, but they only seem to have
`ModeIrregular` set.

This unconditionally clears the bit similar to socket files which should
properly fix the issue on Windows.

Author: jsternberg

cache/contenthash/filehash.go

@@ -41,17 +41,9 @@ func NewFileHash(path string, fi os.FileInfo) (hash.Hash, error) {
 }
 
 func NewFromStat(stat *fstypes.Stat) (hash.Hash, error) {
-	// Clear the irregular file bit if this is some kind of special
-	// file. Pre-Go 1.23 behavior would only add the irregular file
-	// bit to regular files with non-handled reparse points.
-	// Current versions of Go now apply them to directories too.
-	// archive/tar.FileInfoHeader does not handle the irregular bit.
-	if stat.Mode&uint32(os.ModeType&^os.ModeIrregular) != 0 {
-		stat.Mode &^= uint32(os.ModeIrregular)
-	}
-
-	// Clear the socket bit since archive/tar.FileInfoHeader does not handle it
+	// Clear the socket and irregular bits since archive/tar.FileInfoHeader do not handle them
 	stat.Mode &^= uint32(os.ModeSocket)
+	stat.Mode &^= uint32(os.ModeIrregular)
 
 	fi := &statInfo{stat}
 	hdr, err := tar.FileInfoHeader(fi, stat.Linkname)