[PATCH 0/2] dropping manual malloc calculations

While working on the skip_prefix series, I ended up grepping for:

  + strlen("

to find spots in need of skip_prefix. Of course, it turns up many other
nasty ad-hoc calculations. Here's a short series that addresses a few.
There are many more, but hopefully the first patch provides a tool that
can help us in the future.

  [1/2]: strbuf: add xstrdup_fmt helper
  [2/2]: use xstrdup_fmt in favor of manual size calculations

-Peff