Update DustMite

Update DustMite, from the upstream commit cb0855d (+ the backported
fix) to the upstream commit ab68756.

* 6634109 splitter: Add null split mode
* 1f40c63 dustmite: Add --reject
* bba58a4 dustmite: Add --dump-json
* ca93991 dustmite: Fix a placeholder variable name in --help
* 4e38836 dustmite: Add --json
* e504ccc dustmite: Remove use of newer D features
* b173b89 Add support for reading from stdin
* bb73f5e tests: Run dustmite in the test's directory
* f4d2fa7 Remove the extension stripping for single-file input
* 5b5e261 Move JSON loading into splitter module
* 1bf5b4b dustmite: Print informational messages to standard error
* 61b41ae splitter: Don't replace \ with / on POSIX
* 461271f dustmite: Allow omitting the mask portion of --split
* faee9d3 splitter: Refactor
* fd28eb1 splitter: Fix compilation on 32 bits
* 67e676f dustmite: Add --temp-dir
* f594d0a dustmite: More explicit File finalization in DiskWriter
* 372b4a0 dustmite: Fix stdin on MSVCRT
* 4e4a303 README: Update badges
* 9696d83 README: Add illustration
* 56cdd01 README: Show CI status for master branch
* 4da8847 Use skipOver
* b8703eb polyhash: Update GDC assembler syntax
* fd8c598 fuzz: Cast allAddresses length to float
* 62f7899 dustmite: Fix lookback and pingpong strategies reducing to empty set
* bc5f5d6 splitter: Print used splitter when loading files
* c855596 Refactor Entity file properties into separate structure
* 190c180 dustmite: Refactor look-ahead thread handling
* 07ba696 dustmite: Handle errors
* 71362ea Preserve file attributes
* ed78a2e Add support for symbolic links
* 675c757 dustmite: Remove explicit Writer destructors
* ec5c25d Preserve file timestamps
* 7482c9d Preserve directories (incl. empty directories) and their attributes
* 4688772 splitter: Add lisp splitter
* 5b65ade dustmite: Add some assertions
* d0af4a5 splitter: Don't try to reduce module names in D source code
* 178f916 splitter: Smarter diff splitting
* 5f2ac60 splitter: Add debug check to ensure splitting preserves contents
* a093fae splitter: Fix firstHead
* 00c2400 splitter: Improve indent splitter
* 1c54227 splitter: Refactor parseIndent
* ab68756 dustmite: Add --man