Fix and generalize version sort reordering

After having slept on it a few (erm...) more times...

At the risk of overthinking it: I think it would be more correct to say that the earliest starting matching prerelease suffix should determine the sorting order in the first place.

And when there happen to be more than one matching suffixes starting at the same earliest position, then we should pick the longest of them. The new patch 6/7 implements this behavior.

Changes since v1:
- Documentation, in-code comment and commit message updates.
- Use different tagnames and suffixes in the tests, keeping the new tests simpler and changes to existing tests smaller.
- Added a test of questionable value to try to check that we don't read memory before the tagname strings in case of an unusually long suffix.
- Removed unnecessary {}.
- Added two patches on top to address the corner cases and to introduce 'versionsort.suffix' for the resulting more general suffix reordering behavior.

The interdiff at the bottom is between v1 and only the first five patches of v2, i.e. not including the two new patches. I think it's easier to judge the changes this way.

SZEDER Gábor (7):
t7004-tag: delete unnecessary tags with test_when_finished
t7004-tag: use test_config helper
t7004-tag: add version sort tests to show prerelease reordering issues
versioncmp: pass full tagnames to swap_prereleases()
versioncmp: cope with common part overlapping with prerelease suffix
versioncmp: use earliest-longest contained suffix to determine sorting
order
versioncmp: generalize version sort suffix reordering

 Documentation/config.txt  |  44 ++++++++++++----
 Documentation/git-tag.txt |   4 +-
 t/t7004-tag.sh            | 132 +++++++++++++++++++++++++++++++++++++++-------
 versioncmp.c              |  68 +++++++++++++++++-------
 4 files changed, 196 insertions(+), 52 deletions(-)

2.11.0.78.g5a2d011

index 0bcb6790d..c1a9616e9 100644
@@ -3008,8 +3008,12 @@ versionsort.prereleaseSuffix::
 This variable can be specified multiple times, once per suffix. The
 order of suffixes in the config file determines the sorting order
 (e.g. if "-pre" appears before "-rc" in the config file then 1.0-preXX
-is sorted before 1.0-rcXX). The sorting order between different
-suffixes is undefined if they are in multiple config files.
+is sorted before 1.0-rcXX).
+If more than one suffixes match the same tagname, then that tagname will
+be sorted according to the matching suffix which comes first in the
+configuration.
+The sorting order between different suffixes is undefined if they are
+in multiple config files.
 
 web.browser::
 	Specify a web browser that may be used by some commands.

index 87b49a622..f86ac562e 100644
@@ -26,16 +26,15 @@ static int initialized;
 
 /*
  * off is the offset of the first different character in the two strings
- * s1 and s2. If either s1 or s2 contains a prerelease suffix starting
- * at that offset or the character at that offset is part of a
- * prerelease suffix, then that string will be forced to be on top.
+ * s1 and s2. If either s1 or s2 contains a prerelease suffix containing
+ * that offset, then that string will be forced to be on top.
  *
- * If both s1 and s2 contain a (different) suffix at that position, the
- * order is determined by config file.
- *
- * Note that we don't have to deal with the situation when both s1 and
- * s2 contain the same suffix because the common part is already
- * consumed by the caller.
+ * If both s1 and s2 contain a (different) suffix around that position,
+ * their order is determined by the order of those two suffixes in the
+ * configuration.
+ * If any of the strings contains more than one different suffixes around
+ * that position, then that string is sorted according to the contained
+ * suffix which comes first in the configuration.
  *
  * Return non-zero if *diff contains the return value for versioncmp()
  */

@@ -53,18 +52,16 @@ static int swap_prereleases(const char *s1,
 			start = off - suffix_len + 1;
 		else
 			start = 0;
-		for (j = start; j <= off; j++) {
+		for (j = start; j <= off; j++)
 			if (i1 == -1 && starts_with(s1 + j, suffix)) {
 				i1 = i;
 				break;
 			}
-		}
-		for (j = start; j <= off; j++) {
+		for (j = start; j <= off; j++)
 			if (i2 == -1 && starts_with(s2 + j, suffix)) {
 				i2 = i;
 				break;
 			}
-		}
 	}
 	if (i1 == -1 && i2 == -1)
 		return 0;