dirstate-v2: Replace the 32-bit `mode` field with two bits

Authored by SimonSapin.


dirstate-v2: Replace the 32-bit mode field with two bits

Previously we stored the entire value from stat_result.st_mode,
like dirstate-v1 does. However only the executable permission
and type of file (only symbolic links and normal files are supported)
are relevant to Mecurial.

So replace this field with two bits in the existing bitfield byte.
For now the unused space is left as padding, as it will be used
for something else soon.

Differential Revision: https://phab.mercurial-scm.org/D11635