versioncmp: cope with common part overlapping with prerelease suffix

versioncmp: cope with common part overlapping with prerelease suffix

Version sort with prerelease reordering sometimes puts tagnames in the
wrong order, when the common part of two compared tagnames overlaps
with the leading character(s) of one or more configured prerelease
suffixes.  Note the position of "v2.1.0-beta-1":

  $ git -c versionsort.prereleaseSuffix=-beta \
        tag -l --sort=version:refname v2.1.*
  v2.1.0-beta-2
  v2.1.0-beta-3
  v2.1.0
  v2.1.0-RC1
  v2.1.0-RC2
  v2.1.0-beta-1
  v2.1.1
  v2.1.2

The reason is that when comparing a pair of tagnames, first
versioncmp() looks for the first different character in a pair of
tagnames, and then the swap_prereleases() helper function looks for a
configured prerelease suffix _starting at_ that character.  Thus, when
in the above example the sorting algorithm happens to compare the
tagnames "v2.1.0-beta-1" and "v2.1.0-RC2", swap_prereleases() tries to
match the suffix "-beta" against "beta-1" to no avail, and the two
tagnames erroneously end up being ordered lexicographically.

To fix this issue change swap_prereleases() to look for configured
prerelease suffixes _containing_ the position of that first different
character.

Care must be taken, when a configured suffix is longer than the
tagnames' common part up to the first different character, to avoid
reading memory before the beginning of the tagnames.  Add a test that
uses an exceptionally long prerelease suffix to check for this, in the
hope that in case of a regression the illegal memory access causes a
segfault in 'git tag' on one of the commonly used platforms (the test
happens to pass successfully on my Linux system with the safety check
removed), or at least makes valgrind complain.

Under some circumstances it's possible that more than one prerelease
suffixes can be found in the same tagname around that first different
character.  With this simple bugfix patch such a tagname is sorted
according to the contained suffix that comes first in the
configuration for now.  This is less than ideal in some cases, and the
following patch will take care of those.

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