discovery: only calculate closed branches if required
The number of new closed branches is required for printing in error message. So
let's only calculate them if we need to print error about new branches.
Differential Revision: https://phab.mercurial-scm.org/D6314
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.