verify: add a config option to skip certain flag processors
Previously, "hg verify" verifies everything, which could be undesirable when
there are expensive flag processor contents.
This patch adds a "verify.skipflags" developer config. A flag processor will
be skipped if (flag & verify.skipflags) == 0.
In the LFS usecase, that means "hg verify --config verify.skipflags=8192"
will not download all LFS blobs, which could be too large to be stored
locally.
Note: "renamed" is also skipped since its default implementation may call
filelog.data() which will trigger the flag processor.