help: add a mechanism to change flags' help depending on config It seems reasonable to have a similar mechanism for the rest of the help, but no such thing is implemented. The goal is to make the help of commands clearer in the presence of significant default changes, like tweakdefaults or with company-wide hgrcs. In these cases, a user looking at the help of a command doesn't exactly know what his hgrc is doing. Apply to this to the --git option of commands that display diffs, as this option in particular causes confusion for some reason. Differential Revision: https://phab.mercurial-scm.org/D8100