buffer: rely on `GITERR_OOM` set by `git_buf_try_grow`

buffer: rely on `GITERR_OOM` set by `git_buf_try_grow`

The function `git_buf_try_grow` consistently calls `giterr_set_oom`
whenever growing the buffer fails due to insufficient memory being
available. So in fact, we do not have to do this ourselves when a call
to any buffer-growing function has failed due to an OOM situation. But
we still do so in two functions, which this patch cleans up.