narrow: add '--import-rules' flag to tracked command This patch adds a `--import-rules` flag to tracked command provided by narrow extension. Using the --import-rules flag, you can pass a filename from which narrowspecs should be read and added to main narrowspec. A lot of times, in automation or manually also, when you are working with big repo, specifying each path name on commandline using '--addinclude' and '--addexclude' is tedious and something which can scale. So we needed something where we can pass a file to extend the narrowspecs. Nice thing about this is that the automations which reads some file to change the sparse profile, can now read the same file for changing narrowspecs too. Tests are added for the new feature. Differential Revision: https://phab.mercurial-scm.org/D4125