read_branches_file: simplify string handling

read_branches_file: simplify string handling

This function does a lot of manual string handling, and has
some unnecessary limits. This patch cleans up a number of
things:

  1. Drop the arbitrary 1000-byte limit on the size of the
     remote name (we do not have such a limit in any of the
     other remote-reading mechanisms).

  2. Replace fgets into a fixed-size buffer with a strbuf,
     eliminating any limits on the length of the URL.

  3. Replace manual whitespace handling with strbuf_trim
     (since we now have a strbuf). This also gets rid
     of a call to strcpy, and the confusing reuse of the "p"
     pointer for multiple purposes.

  4. We currently build up the refspecs over multiple strbuf
     calls. We do this to handle the fact that the URL "frag"
     may not be present. But rather than have multiple
     conditionals, let's just default "frag" to "master".
     This lets us format the refspecs with a single xstrfmt.
     It's shorter, and easier to see what the final string
     looks like.

     We also update the misleading comment in this area (the
     local branch is named after the remote name, not after
     the branch name on the remote side).

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