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