abbrev: prepare for new world order

The code that sets custom abbreviation length, in response to
command line argument, often does something like this:

	if (skip_prefix(arg, "--abbrev=", &arg))
		abbrev = atoi(arg);
	else if (!strcmp("--abbrev", &arg))
		abbrev = DEFAULT_ABBREV;
	/* make the value sane */
	if (abbrev < 0 || 40 < abbrev)
		abbrev = ... some sane value ...

However, it is pointless to sanity-check and tweak the value
obtained from DEFAULT_ABBREV.  We are going to allow it to be
initially set to -1 to signal that the default abbreviation length
must be auto sized upon the first request to abbreviate, based on
the number of objects in the repository, and when that happens,
rejecting or tweaking a negative value to a "saner" one will
negatively interfere with the auto sizing.  The codepaths for

    git rev-parse --short <object>
    git diff --raw --abbrev

do exactly that; allow them to pass possibly negative abbrevs
intact, that will come from DEFAULT_ABBREV in the future.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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