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

--- a/tests/test-help.t	Sat Feb 08 23:39:55 2020 -0500
+++ b/tests/test-help.t	Sun Feb 09 15:50:36 2020 -0500
@@ -788,6 +788,12 @@
   (use 'hg help extensions' for information on enabling extensions)
   [255]
 
+Checking that help adapts based on the config:
+
+  $ hg help diff --config ui.tweakdefaults=true | egrep -e '^ *(-g|config)'
+   -g --[no-]git            use git extended diff format (default: on from
+                            config)
+
 Make sure that we don't run afoul of the help system thinking that
 this is a section and erroring out weirdly.