remote: drop auto-strlen behavior of make_branch() and make_rewrite()

remote: drop auto-strlen behavior of make_branch() and make_rewrite()

The make_branch() and make_rewrite() functions can take a NUL-terminated
string or a ptr/len pair. They use a sentinel value of "0" for the len
to tell the difference between the two. However, when parsing config
like:

  [branch ""]
  merge = whatever

whose key flattens to:

  branch..merge

we might actually have a zero-length branch name. This is obviously
nonsense, but the current code would consider it as a NUL-terminated
string and use the branch name ".merge".

We could use a better sentinel value here (like "-1"), but that gets in
the way of moving to size_t, which is a more appropriate type for a
ptr/len combo.

Let's instead just drop this feature and have the callers (of which
there are only two total) use strlen() themselves. This simplifies the
code, and lets us move to using size_t.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>