usage: add a `usage.repository-role` config

This config will be used for behavior and performance adjustment depending of
the repository role.

mercurial/configitems.toml

@@ -2489,6 +2489,18 b' name = "verbose"'
 default = false
 
 [[items]]
+section = "usage"
+name = "repository-role"
+default = "default"
+documentation = """What this repository is used for.
+
+This is used to adjust behavior and performance to best fit the repository purpose.
+
+Currently recognised values are:
+- default: an all purpose repository
+"""
+
+[[items]]
 section = "verify"
 name = "skipflags"
 default = 0

mercurial/helptext/config.txt

@@ -2994,6 +2994,18 b' User interface controls.'
     Increase the amount of output printed. (default: False)
 
 
+``usage``
+---------
+
+``repository-role``
+    What this repository is used for.
+
+    This is used to adjust behavior and performance to best fit the repository purpose.
+
+    Currently recognised values are:
+    - default: an all purpose repository
+
+
 ``command-templates``
 ---------------------