vfs: extract 'vfs' class and related code to a new 'vfs' module (API) The 'scmutil' is growing large (1500+ lines) and 2/5 of it is related to vfs. We extract the 'vfs' related code in its own module get both module back to a better scale and clearer contents. We keep all the references available in 'scmutil' for now as many reference needs to be updated.

vfs: replace 'scmutil.opener' usage with 'scmutil.vfs' The 'vfs' class is the first class citizen for years. We remove all usages of the older API. This will let us remove the old API eventually.

fsmonitor: remove use of repo.opener This has been deprecated, so we need to switch to the appropriate vfs apis.

help: align description of 'base rev' with reality [issue5488] The text about revlogs seems to be wrong about -1 being used to indicate the start of a delta chain. Attempt to correct this.