fast-import: directly use strbufs for paths

fast-import: directly use strbufs for paths

Previously, one case would not write the path to the strbuf: when the
path is unquoted and at the end of the string. It was essentially
copy-on-write. However, with the logic simplification of the previous
commit, this case was eliminated and the strbuf is always populated.

Directly use the strbufs now instead of an alias.

Since this already changes all the lines that use the strbufs, rename
them from `uq` to be more descriptive. That they are unquoted is not
their most important property, so name them after what they carry.

Additionally, `file_change_m` no longer needs to copy the path before
reading inline data.

Signed-off-by: Thalia Archibald <thalia@archibald.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>