remotenames: add functionality to store remotenames under .hg/hgremotenames/ This patch moves the functionality from remotenames extension to store remotenames to core. Storage format used by remotenames extension: A single file `.hg/remotenames` with an entry in each line where each line is of format: `node nametype remotepath/name` where nametype is either 'bookmarks' or 'branches'. This was not the best way to store data, so while moving to core the storage format was changed but yet not the final format. The storage format used by core after this patch will be: * A file for each type of name i.e. bookmarks and branches in .hg/remotenames/ directory * A version number on the top of the file. The version for current format is 0. * An entry in each line where each line is of the format `node\0remotepath\0name` The logic to sync with existing remotenames file and saving journals and other related things will be moved to core in next patches incrementally. Thanks to Ryan, Augie and Durham for suggestions on storage format. Previously reviewed as D939. Differential Revision: https://phab.mercurial-scm.org/D1548