upgrade: demonstrate that a no-op upgrade still performs everything

An upgrade operation which is not adding or removing anything should ideally
error out. However, it does the whole cloning and everything. Next patch will
fix it.

Differential Revision: https://phab.mercurial-scm.org/D9694

tests/test-upgrade-repo.t

@@ -1644,3 +1644,42 b' upgrade from hgrc'
   sparserevlog
   store
   $ hg debugsidedata -c 0
+
+Demonstrate that nothing to perform upgrade will still run all the way through
+FIXME: this should return early
+
+  $ hg debugupgraderepo --run
+  upgrade will perform the following actions:
+  
+  requirements
+     preserved: dotencode, exp-sidedata-flag, fncache, generaldelta, revlog-compression-zstd, revlogv1, sparserevlog, store
+  
+  processed revlogs:
+    - all-filelogs
+    - changelog
+    - manifest
+  
+  beginning upgrade...
+  repository locked and read-only
+  creating temporary repository to stage upgraded data: $TESTTMP/sparserevlogrepo/.hg/upgrade.* (glob)
+  (it is safe to interrupt this process any time before data migration completes)
+  migrating 3 total revisions (1 in filelogs, 1 in manifests, 1 in changelog)
+  migrating 297 bytes in store; 103 bytes tracked data
+  migrating 1 filelogs containing 1 revisions (64 bytes in store; 0 bytes tracked data)
+  finished migrating 1 filelog revisions across 1 filelogs; change in size: 0 bytes
+  migrating 1 manifests containing 1 revisions (110 bytes in store; 45 bytes tracked data)
+  finished migrating 1 manifest revisions across 1 manifests; change in size: 0 bytes
+  migrating changelog containing 1 revisions (123 bytes in store; 58 bytes tracked data)
+  finished migrating 1 changelog revisions; change in size: 0 bytes
+  finished migrating 3 total revisions; total change in store size: 0 bytes
+  copying phaseroots
+  data fully upgraded in a temporary repository
+  marking source repository as being upgraded; clients will be unable to read from repository
+  starting in-place swap of repository data
+  replaced files will be backed up at $TESTTMP/sparserevlogrepo/.hg/upgradebackup.* (glob)
+  replacing store...
+  store replacement complete; repository was inconsistent for *s (glob)
+  finalizing requirements file and making repository readable again
+  removing temporary repository $TESTTMP/sparserevlogrepo/.hg/upgrade.* (glob)
+  copy of old repository backed up at $TESTTMP/sparserevlogrepo/.hg/upgradebackup.* (glob)
+  the old repository will not be deleted; remove it to free up disk space once the upgraded repository is verified