msvc: avoid using minus operator on unsigned types

MSVC complains about this with `-Wall`, which can be taken as a sign
that this is indeed a real bug. The symptom is:

	C4146: unary minus operator applied to unsigned type, result
	still unsigned

Let's avoid this warning in the minimal way, e.g. writing `-1 -
<unsigned value>` instead of `-<unsigned value> - 1`.

Note that the change in the `estimate_cache_size()` function is
needed because MSVC considers the "return type" of the `sizeof()`
operator to be `size_t`, i.e. unsigned, and therefore it cannot be
negated using the unary minus operator.

Even worse, that arithmetic is doing extra work, in vain. We want to
calculate the entry extra cache size as the difference between the
size of the `cache_entry` structure minus the size of the
`ondisk_cache_entry` structure, padded to the appropriate alignment
boundary.

To that end, we start by assigning that difference to the `per_entry`
variable, and then abuse the `len` parameter of the
`align_padding_size()` macro to take the negative size of the ondisk
entry size. Essentially, we try to avoid passing the already calculated
difference to that macro by passing the operands of that difference
instead, when the macro expects operands of an addition:

	#define align_padding_size(size, len) \
		((size + (len) + 8) & ~7) - (size + len)

Currently, we pass A and -B to that macro instead of passing A - B and
0, where A - B is already stored in the `per_entry` variable, ready to
be used.

This is neither necessary, nor intuitive. Let's fix this, and have code
that is both easier to read and that also does not trigger MSVC's
warning.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

read-cache.c

@@ -1276,7 +1276,7 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 	 */
 	if (istate->cache_nr > 0 &&
 		strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
-		pos = -istate->cache_nr - 1;
+		pos = -1 - istate->cache_nr;
 	else
 		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
 
@@ -1906,7 +1906,7 @@ static size_t estimate_cache_size(size_t ondisk_size, unsigned int entries)
 	/*
 	 * Account for potential alignment differences.
 	 */
-	per_entry += align_padding_size(sizeof(struct cache_entry), -sizeof(struct ondisk_cache_entry));
+	per_entry += align_padding_size(per_entry, 0);
 	return ondisk_size + entries * per_entry;
 }
 

sha1-lookup.c

@@ -97,7 +97,7 @@ int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
 			lo = mi + 1;
 		mi = lo + (hi - lo) / 2;
 	} while (lo < hi);
-	return -lo-1;
+	return -1 - lo;
 }
 
 int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo,