translation stream does not expand keywords on partial reads

From #svn-dev:

<stsp> svn_subst_stream_translated() does not expand keywords if they're not
read in their entirety by the caller
<stsp> is that a bug?
<stsp> i would expect that, if I have the input containing a keyword such as
One$MyKeyword$Two, which expands to "my keyword expanded", and I read up to the
position of the character K, the result would be "Onemy"
<stsp> but what is actually returned from svn_stream_read is One$My
<stsp> that makes setting a mark within a keyword rather pointless
<julianf> stsp: ew, that's an ugly bug in stream_translated.
<julianf> That's just waiting to blow up.
<stsp> julianf, yeah I think it should read ahead, expand the keyword, and
return part of the expansion
<julianf> +1

See also r936844 which is related.