manifestcache: only lock the repository if the debug command touch the cache

Not doing so had two consequences:
1) the command cannot be run on read only repositories,
2) when using --add on an empty cache, the command crash prematurely trying to
   read the cache file on disk.

mercurial/debugcommands.py

@@ -1465,26 +1465,31 def debuglocks(ui, repo, **opts):
     ], '')
 def debugmanifestfulltextcache(ui, repo, add=None, **opts):
     """show, clear or amend the contents of the manifest fulltext cache"""
-    with repo.lock():
+
+    def getcache():
         r = repo.manifestlog.getstorage(b'')
         try:
-            cache = r._fulltextcache
+            return r._fulltextcache
         except AttributeError:
-            ui.warn(_(
-                "Current revlog implementation doesn't appear to have a "
-                'manifest fulltext cache\n'))
-            return
+            msg = _("Current revlog implementation doesn't appear to have a "
+                    "manifest fulltext cache\n")
+            raise error.Abort(msg)
 
         if opts.get(r'clear'):
+        with repo.lock():
+            cache = getcache()
             cache.clear()
 
         if add:
+        with repo.lock():
             try:
-                manifest = repo.manifestlog[r.lookup(add)]
+                m = repo.manifestlog
+                manifest = m[m.getstorage(b'').lookup(add)]
             except error.LookupError as e:
                 raise error.Abort(e, hint="Check your manifest node id")
             manifest.read()  # stores revisision in cache too
 
+    cache = getcache()
         if not len(cache):
             ui.write(_('cache empty\n'))
         else:

tests/test-manifest.t

@@ -107,3 +107,15 Showing the content of the caches after
 
   $ hg debugmanifestfulltextcache
   cache empty
+
+Adding a new persistent entry in the cache
+
+  $ hg debugmanifestfulltextcache --add 1e01206b1d2f72bd55f2a33fa8ccad74144825b7
+  cache contains 1 manifest entries, in order of most to least recent:
+  id: 1e01206b1d2f72bd55f2a33fa8ccad74144825b7, size 133 bytes
+  total cache data size 157 bytes, on-disk 157 bytes
+
+  $ hg debugmanifestfulltextcache
+  cache contains 1 manifest entries, in order of most to least recent:
+  id: 1e01206b1d2f72bd55f2a33fa8ccad74144825b7, size 133 bytes
+  total cache data size 157 bytes, on-disk 157 bytes