[BUG] add_again() off-by-one error in custom format

In a custom pretty format, using the '+' or ' ' combinators to prefix
a non-empty expansion with whitespace will erroneously truncate
subsequent expansions of the same type.

Normally '%+X' inserts a newline before <X>, IFF the expansion of X is
non-empty:

    $ git log -n 1 --pretty='format:newline:%+an'
    newline:
    MyName

For the abbreviated commit hash placeholder ('h'), pretty.c uses
add_again() to cache the result of the expansion, and then uses that
result in future expansions. This causes problems when the expansion
includes whitespace:

    $ git log -n 1 --pretty='format:newline:%+h%nno_newline:%h'
    newline:
    a0b1c2d
    no_newline:
    a0b1c2

The second expansion of the hash added an unwanted newline and removed
the final character. It seemingly used the cached expansion *starting
from the inserted newline*.

The expected output is:

    $ git log -n 1 --pretty='format:newline:%+h%nno_newline:%h'
    newline:
    a0b1c2d
    no_newline:a0b1c2d


/CC René from commit b9c62321 and Junio from 9fa708d