format: create new repository as 'generaldelta' by default

Since we have pushed back the performance issue related to general delta behind
another configuration (Still off by default), we can safely create new
repository with general delta support. As client are compatible with it since
Mercurial 1.9 (4.5 years ago) I do no expect any significant compatibility
issues.

mercurial/help/config.txt

@@ -670,9 +670,9 b' Example for ``~/.hgrc``::'
     Enable or disable the "generaldelta" repository format which improves
     repository compression by allowing "revlog" to store delta against arbitrary
     revision instead of the previous stored one. This provides significant
-    improvement for repositories with branches. Disabling this option ensures that
-    the on-disk format of newly created repository will be compatible with
-    Mercurial before version 1.9.
+    improvement for repositories with branches. Enabled by default. Disabling
+    this option ensures that the on-disk format of newly created repository will
+    be compatible with Mercurial before version 1.9.
 
 ``usestore``
     Enable or disable the "store" repository format which improves

mercurial/scmutil.py

@@ -1176,7 +1176,7 b' def gdinitconfig(ui):'
     """
     # experimental config: format.generaldelta
     return (ui.configbool('format', 'generaldelta', False)
-            or ui.configbool('format', 'usegeneraldelta', False))
+            or ui.configbool('format', 'usegeneraldelta', True))
 
 def gddeltaconfig(ui):
     """helper function to know if incoming delta should be optimised