context: safegaurd against 'lx' being passed as file flag in manifest

Subversion can have a file as executable link. When using hgsubversion, we will
have both islink and isexec True. This will lead to _flags being set to `lx`.
However, manifest expects flag to be one-byte so it will crash if 'lx' is
passed. Also it's impossible to have an executable link.

This patch will safegaurd us from having 'lx' being a possible value.

This was authored by Ivan Lezhankin from Yandex.

Differential Revision: https://phab.mercurial-scm.org/D3985

mercurial/context.py

@@ -2327,7 +2327,12 b' class memfilectx(committablefilectx):'
         revision being committed, or None."""
         super(memfilectx, self).__init__(repo, path, None, changectx)
         self._data = data
-        self._flags = (islink and 'l' or '') + (isexec and 'x' or '')
+        if islink:
+            self._flags = 'l'
+        elif isexec:
+            self._flags = 'x'
+        else:
+            self._flags = ''
         self._copied = None
         if copied:
             self._copied = (copied, nullid)