rust: replace most "operation" structs with functions The hg-core crate has a partially-formed concept of "operation", represented as structs with constructors and a `run` method. Each struct’s contructor takes different parameters, and each `run` has a different return type. Constructors typically don’t do much more than store parameters for `run` to access them. There was a comment about adding an `Operation` trait when the language supports expressing something so general, but it’s hard to imagine how operations with such different APIs could be used in a generic context. This commit starts removing the concept of "operation", since those are pretty much just functions. Differential Revision: https://phab.mercurial-scm.org/D9595

rust: change &PathBuf parameters to &Path This is just as useful in function bodies, and a less strict requirement for callers. Differential Revision: https://phab.mercurial-scm.org/D9594

extdiff: add --from/--to and deprecate -r, as was done for `hg diff` I recently replaced `hg diff`'s `-r` flag by `--from` and `--to` and then deprecated the `-r` flag. This patch repeats that for `hg extdiff`. Differential Revision: https://phab.mercurial-scm.org/D9624

extdiff: fix crash when showing diff from wdir() Differential Revision: https://phab.mercurial-scm.org/D9623