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.