scmutil: add method for looking up a context given a revision symbol changectx's constructor currently supports a mix if inputs: * integer revnums * binary nodeids * '.', 'tip', 'null' * stringified revnums * namespaced identifiers (e.g. bookmarks and tags) * hex nodeids * partial hex nodeids The first two are always internal [1]. The other five can be specified by the user. The third type ('.', 'tip', 'null') often comes from either the user or internal callers. We probably have some internal callers that pass hex nodeids too, perhaps even partial ones (histedit?). There are only a few callers that pass user-supplied strings: revsets.stringset, peer.lookup, webutil.changeidctx, and maybe one or two more. Supporting this mix of things in the constructor is convenient, but a bit strange, IMO. For example, if repo[node] is given a node that's not in the repo, it will first check if it's bookmark etc before raising an exception. Of course, the risk of it being a bookmark is extremely small, but it just feels ugly. Also, a problem with having this code in the constructor (whether it supports a mix of types or not) is that it's harder to override (I'd like to override it, and that's how this series started). This patch starts moving out the handling of user-supplied strings by introducing scmutil.revsymbol(). So far, that just checks that the input is indeed a string, and then delegates to repo[symbol]. The patch also calls it from revsets.stringset to prove that it works. [1] Well, you probably can enter a 20-byte binary nodeid on the command line, but I don't think we should care to preserve support for that. Differential Revision: https://phab.mercurial-scm.org/D3024