versioncmp: pass full tagnames to swap_prereleases()

The swap_prereleases() helper function is responsible for finding
configured prerelease suffixes in a pair of tagnames to be compared,
but this function currently gets to see only the parts of those two
tagnames starting at the first different character.  To fix some
issues related to the common part of two tagnames overlapping with
leading part of a prerelease suffix, this helper function must see
both full tagnames.

In preparation for the fix in the following patch, refactor
swap_prereleases() and its caller to pass two full tagnames and an
additional offset indicating the position of the first different
character.

While updating the comment describing that function, remove the
sentence about not dealing with both tagnames having the same suffix.
Currently it doesn't add much value (we know that there is a different
character, so it's obvious that it can't possibly be the same suffix
in both), and at the end of this patch series it won't even be true
anymore.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

versioncmp.c

@@ -25,32 +25,28 @@ static const struct string_list *prereleases;
 static int initialized;
 
 /*
- * p1 and p2 point to the first different character in two strings. If
+ * off is the offset of the first different character in the two strings
- * either p1 or p2 starts with a prerelease suffix, it will be forced
+ * s1 and s2. If either s1 or s2 contains a prerelease suffix starting
- * to be on top.
+ * at that offset, it will be forced to be on top.
  *
- * If both p1 and p2 start with (different) suffix, the order is
+ * If both s1 and s2 contain a (different) suffix at that position,
- * determined by config file.
+ * their order is determined by the order of those two suffixes in the
- *
+ * configuration.
- * Note that we don't have to deal with the situation when both p1 and
- * p2 start with the same suffix because the common part is already
- * consumed by the caller.
  *
  * Return non-zero if *diff contains the return value for versioncmp()
  */
-static int swap_prereleases(const void *p1_,
+static int swap_prereleases(const char *s1,
-			    const void *p2_,
+			    const char *s2,
+			    int off,
 			    int *diff)
 {
-	const char *p1 = p1_;
-	const char *p2 = p2_;
 	int i, i1 = -1, i2 = -1;
 
 	for (i = 0; i < prereleases->nr; i++) {
 		const char *suffix = prereleases->items[i].string;
-		if (i1 == -1 && starts_with(p1, suffix))
+		if (i1 == -1 && starts_with(s1 + off, suffix))
 			i1 = i;
-		if (i2 == -1 && starts_with(p2, suffix))
+		if (i2 == -1 && starts_with(s2 + off, suffix))
 			i2 = i;
 	}
 	if (i1 == -1 && i2 == -1)
@@ -121,7 +117,8 @@ int versioncmp(const char *s1, const char *s2)
 		initialized = 1;
 		prereleases = git_config_get_value_multi("versionsort.prereleasesuffix");
 	}
-	if (prereleases && swap_prereleases(p1 - 1, p2 - 1, &diff))
+	if (prereleases && swap_prereleases(s1, s2, (const char *) p1 - s1 - 1,
+					    &diff))
 		return diff;
 
 	state = result_type[state * 3 + (((c2 == '0') + (isdigit (c2) != 0)))];