0644e6f Fix copying properties in 'format' when it produces padding

[Mark Oteiza]

Hi Eli,

Since this change, both eldoc and isearch appear to be broken--haven't
found further effects yet.  In particular, it appears that the face
property at the beginning of the string is being applied to the entire
string indiscriminately.

If I'm understanding Bug#23859 and friends, this behavior suggests that
eldoc and isearch need to be fixed, correct?  I am not certain, so
I wanted to ask.