mailmap: avoid allocation when reading from blob

The read_mailmap_blob function reads the whole blob into a
buffer, then calls read_mailmap_buf to do the heavy lifting.
The latter function ends up making a NUL-terminated copy of
each line of the blob in order to call read_mailmap_line,
which was originally written to handle the line-at-a-time
input from fgets().

We can avoid the extra copy if we simply NUL-terminate each
line in place, and assume that the input buffer is itself
NUL-terminated. Neither of these is a problem, since our
only caller is read_mailmap_blob, which has a non-const
NUL-terminated buffer already.