lib/repo-commit: Fix types of content size cache entries

lib/repo-commit: Fix types of content size cache entries

Use goffset rather than gsize for file sizes. More importantly, get the
unpacked_size from g_file_info_get_size() (goffset) rather than from the
splice return value, which has type gssize.

This will make a difference on 32-bit systems, where goffset is defined
as off64_t, but gsize is 32 bits.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #999
Approved by: cgwalters