Due to a couple regressions from version 3.1, version 3.1.1 has been released to clean up the regressions. Apologies for the delay in getting this out as I know the hg issue in particular has been painful for people hitting it.
The two primary changes are:
* Fix for hg.exe being unable to load the correct Python DLL on some systems, causing various commands (such as ./mach bootstrap) to fail that tried to invoke it.
* The fsmonitor extension was re-disabled due to significant performance regressions for some common operations caused by watchman excessively polling the filesystem.
If you're already using version 3.1 without issue, you probably don't need to pick up this release. You may want to consider disabling fsmonitor in the mean time, however. You can do that by making the change below to <mozilla-build>/python/mercurial.ini: