statichttprepo: don't modify localrepo class variables

This happened to crash in subrepo setups when the base class variable was
modified twice.

Now we don't modify the class variable but an instance copy.

mercurial/statichttprepo.py

@@ -128,7 +128,7 b' class statichttprepository(localrepo.loc'
         self._branchcachetip = None
         self.encodepats = None
         self.decodepats = None
-        self.capabilities.remove("pushkey")
+        self.capabilities = self.capabilities.difference(["pushkey"])
 
     def url(self):
         return self._url

tests/test-static-http.t

@@ -102,27 +102,36 b' trying clone -r'
   updating to branch default
   2 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
-test with "/" URI (issue 747)
+test with "/" URI (issue 747) and subrepo
 
   $ hg init
+  $ hg init sub
+  $ hg -R sub tag not-empty
+  $ echo sub=sub > .hgsub
   $ echo a > a
-  $ hg add a
-  $ hg ci -ma
+  $ hg add a .hgsub
+  $ hg -q ci -ma
   $ hg clone static-http://localhost:$HGPORT/ local2
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
-  added 1 changesets with 1 changes to 1 files
+  added 1 changesets with 3 changes to 3 files
   updating to branch default
-  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  pulling subrepo sub from static-http://localhost:$HGPORT/sub
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd local2
   $ hg verify
   checking changesets
   checking manifests
   crosschecking files in changesets and manifests
   checking files
-  1 files, 1 changesets, 1 total revisions
+  3 files, 1 changesets, 3 total revisions
   $ cat a
   a
   $ hg paths