bookmarks: make _readactive safe when readlines raises ENOENT

When reading over static http, the file isn't actually opened until
the readlines() call, so we have to check for ENOENT IOErrors here
too. This is necessary so that we can use the bmstore everywhere for
managing the active bookmark, which will be true in the next change.

mercurial/bookmarks.py

@@ -165,10 +165,19 b' def readactive(repo):'
             raise
         return None
     try:
-        # No readline() in osutil.posixfile, reading everything is cheap
+        # No readline() in osutil.posixfile, reading everything is
+        # cheap.
+        # Note that it's possible for readlines() here to raise
+        # IOError, since we might be reading the active mark over
+        # static-http which only tries to load the file when we try
+        # to read from it.
         mark = encoding.tolocal((file.readlines() or [''])[0])
         if mark == '' or mark not in repo._bookmarks:
             mark = None
+    except IOError as inst:
+        if inst.errno != errno.ENOENT:
+            raise
+        return None
     finally:
         file.close()
     return mark