SourceTree has an embedded Mercurial that is about a year old. I tried to have it use my system Mercurial, which is version 2.6, and it showed an error message: "Patching Mercurial failed", "SourceTree requires a few small changes to Mercurial to run optimally, but applying one of these patches (hg_patch_prompting_hg1.9.diff) against your system copy failed with the error ''
SourceTree will use its internal copy of Mercurial instead."