Escape spaces in HTTP URLs during replication

If the repository name contains spaces (e.g. "-- All Projects --")
the spaces must be URL encoded using the esacpe "%20" in order to
appear in an http:// or https:// URL.

Encoding does *not* happen for ssh:// and git:// protocols as these
both pass-through the remote repository name as-is.

Change-Id: I3bb494c1bc3e21f3a9960071e9930a5ec1c8cf1a
Signed-off-by: Shawn O. Pearce <sop@google.com>