plumbing/object: avoid O(N^2) string building when decoding commit me…

[dsymonds]

…ssage

Most commits have relatively small messages, so this was never noticeable. However, there are some repositories that have semi-automated messages that can get very large (e.g. github.com/riscv/riscv-clang and its riscv-trunk branch), on the order of 109k lines. Changing from string += to using a bytes.Buffer reduces the time for Commit.Decode for that specific case from 35s to 74ms.

Signed-off-by: David Symonds [email protected]

[dsymonds]

Hey, any chance this'll get merged soon, @mcuadros @smola ?