fold: improve error messages for multiple heads and roots This commit adds hints and i18n to the error messages about non-linear revisions, along with corresponding tests.

fold: reword error message for public commits This rewords the error message when attempting to fold public csets. It is somewhat clearer to not use contractions in formal writing. It also uses i18n for it. This commit also adds a new test for this error message.

fold: overhaul handling of revisions with --rev (BC) The fold command parsed the revision arguments in a very peculiar and idiosyncratic fashion: if revisions were passed without a --rev argument, then they would all be extended to ".", but if they were passed with --rev, then no extension would happen. Passing revisions both with and without --rev would abort. This is inconsistent with the way all other hg commands parse revision arguments. We have several examples of command where several revisions are passed, and the --rev option is optional for specifying those revisions (update, strip, export). This patch alters the way in which fold parses its revision arguments. No distinction is made between revisions passed with or without the --rev argument. Regardless if a single or multiple revision is specified, all will be folded together into the parent of the working directory. If the --exact argument is passed, then the parent of the working directory is ignored and the specified revisions must be a single contiguous line. The docstring and tests are modified accordingly.

fold: emit error message when folding a single revision Previously, a confusing error state would happen if the revision set to fold was a singleton.

fold: clarify message for empty revision set and abort instead (BC) I find it clearer that there are no revisions to fold because none were specified. Otherwise it seems ambiguous: maybe there are no revisions to fold because it's impossible to fold whatever was given? In analogy of other commands that are given no revisions (e.g. strip, graft), this is also now an abort instead of a mere error message, which is a BC.

evolve: do not use _ as an unused variable The _ variable is the i18n function from core Mercurial, so locally assigning to it can lead to name clashes.