sidedatacopies: add a new requirement for storing copies into sidedata

The end goal is to have changesets centric sidedata information stored into
changelog sidedata. This make it possible to use the changeset based copy
tracing algorithm on any repository without affecting hashes.

The actual implementation is coming. The feature is marked as experimental
(do not use in production) until we stabilise details about the format.

Differential Revision: https://phab.mercurial-scm.org/D6945

mercurial/configitems.py

@@ -748,6 +748,12 b' coreconfigitem('
     b'format', b'usestore', default=True,
 )
 coreconfigitem(
+    b'format',
+    b'exp-use-copies-side-data-changeset',
+    default=False,
+    experimental=True,
+)
+coreconfigitem(
     b'format', b'use-side-data', default=False, experimental=True,
 )
 coreconfigitem(

mercurial/localrepo.py

@@ -440,6 +440,10 b" SPARSEREVLOG_REQUIREMENT = b'sparserevlo"
 # information for revision without altering their original hashes.
 SIDEDATA_REQUIREMENT = b'exp-sidedata-flag'
 
+# A repository with the the copies-sidedata-changeset requirement will store
+# copies related information in changeset's sidedata.
+COPIESSDC_REQUIREMENT = b'exp-copies-sidedata-changeset'
+
 # Functions receiving (ui, features) that extensions can register to impact
 # the ability to load repositories with custom requirements. Only
 # functions defined in loaded extensions are called.
@@ -997,6 +1001,7 b' class localrepository(object):'
         b'revlogv1',
         b'generaldelta',
         b'treemanifest',
+        COPIESSDC_REQUIREMENT,
         REVLOGV2_REQUIREMENT,
         SIDEDATA_REQUIREMENT,
         SPARSEREVLOG_REQUIREMENT,
@@ -3512,6 +3517,10 b' def newreporequirements(ui, createopts):'
     # experimental config: format.use-side-data
     if ui.configbool(b'format', b'use-side-data'):
         requirements.add(SIDEDATA_REQUIREMENT)
+    # experimental config: format.exp-use-copies-side-data-changeset
+    if ui.configbool(b'format', b'exp-use-copies-side-data-changeset'):
+        requirements.add(SIDEDATA_REQUIREMENT)
+        requirements.add(COPIESSDC_REQUIREMENT)
     if ui.configbool(b'experimental', b'treemanifest'):
         requirements.add(b'treemanifest')