rust-matchers: remove default implementations for `Matcher` trait
We don't expect a whole lot of matchers to be defined, and this makes it more
obvious what a matcher does by reading its `impl Matcher for FooMatcher`.
This patch has the added benefit of fixing the `AlwaysMatcher`, its `matches`
function differs from the former default.
Differential Revision: https://phab.mercurial-scm.org/D7255
https://bz.mercurial-scm.org/619
$ hg init
$ echo a > a
$ hg ci -Ama
adding a
$ echo b > b
$ hg branch b
marked working directory as branch b
(branches are permanent and global, did you want a bookmark?)
$ hg ci -Amb
adding b
$ hg co -C 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
Fast-forward:
$ hg merge b
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg ci -Ammerge
Bogus fast-forward should fail:
$ hg merge b
abort: merging with a working directory ancestor has no effect
[255]
Even with strange revset (issue4465)
$ hg merge ::.
abort: merging with a working directory ancestor has no effect
[255]