rebase: document that tool does not apply to deleted files

rebase: mention help merge-tools in help

copies: optimize forward copy detection logic for rebases Forward copy detection (i.e. detecting what files have been moved/copied in commit X since ancestor Y) previously required diff'ing the manifests of both X and Y. This was expensive since it required reading both entire manifests and doing a set difference (they weren't already in a set because of the lazymanifest work). This cost almost 1 second on very large repositories, and happens N times for a rebase of N commits. This patch optimizes it for the case of rebase. In a rebase, we are comparing a commit against it's immediate parent, and therefore we can know what files changed by looking at ctx.files(). This lets us drastically decrease the size of the set comparison, and makes it O(# of changes) instead of O(size of manifest). This makes it take 1ms instead of 1000ms.