[PATCH 0/16] skip_prefix refactoring and cleanups

A while ago[1] we discussed refactoring skip_prefix (or adding something like it) to make it more natural to call. This morning I decided to take a look at doing this, and went down a rabbit hole of cleanups. This is part one of the result.

The short of it is that skip_prefix can now be used like this:

  if (skip_prefix(arg, "--foo=", &value)
	handle_foo(value);

or even:

  /* arg remains valid if we didn't match! */
  if (skip_prefix(arg, "--foo=", &arg))
	handle_foo(arg);
  else if (skip_prefix(arg, "--bar", &arg))
	handle_bar(arg);

though the latter form is not always useful if the conditional code wants to see all of "arg".

  [01/16]: parse_diff_color_slot: drop ofs parameter
  [02/16]: daemon: mark some strings as const
  [03/16]: avoid using skip_prefix as a boolean

    These ones are preparatory cleanup.

  [04/16]: refactor skip_prefix to return a boolean

    The actual refactoring; it changes the existing callers[2] at the
    same time.

  [05/16]: apply: use skip_prefix instead of raw addition
  [06/16]: fast-import: fix read of uninitialized argv memory
  [07/16]: transport-helper: avoid reading past end-of-string

    These three are conversions that actually fix bugs.

  [08/16]: use skip_prefix to avoid magic numbers
  [09/16]: use skip_prefix to avoid repeating strings

    These are the straightforward conversions lumped together by the
    problem they are solving.

  [10/16]: fast-import: use skip_prefix for parsing input
  [11/16]: daemon: use skip_prefix to avoid magic numbers
  [12/16]: stat_opt: check extra strlen call
  [13/16]: fast-import: refactor parsing of spaces
  [14/16]: fetch-pack: refactor parsing in get_ack
  [15/16]: git: avoid magic number with skip_prefix

    These ones are variants of the above two that needed extra
    discussion or attention for various reasons.

  [16/16]: use skip_prefix to avoid repeated calculations

    These ones don't solve a specific problem as the patches above do,
    but I think the code ends up more readable.

My conversions are by now means exhaustive. After grepping through all of the starts_with up to about http.c, I decided to call it a day. But we can easily convert more as time goes on.

[1] http://thread.gmane.org/gmane.comp.version-control.git/239438/focus=239564

    I started from scratch this morning, oblivious to the fact that René
    posted something very similar to patch 4 in that thread.

[2] I test-merged with 'pu'. There is a minor textual conflict with
    jk/commit-buffer-length that should be easy to resolve. There's also
    one new caller of skip_prefix added in cc/interpret-trailers. It
    needs this fix when merged: