versioncmp: generalize version sort suffix reordering

versioncmp: generalize version sort suffix reordering

The 'versionsort.prereleaseSuffix' configuration variable, as its name
suggests, is supposed to only deal with tagnames with prerelease
suffixes, and allows sorting those prerelease tags in a user-defined
order before the suffixless main release tag, instead of sorting them
simply lexicographically.

However, the previous changes in this series resulted in an
interesting and useful property of version sort:

  - The empty string as a configured suffix matches all tagnames,
    including tagnames without any suffix, but

  - tagnames containing a "real" configured suffix are still ordered
    according to that real suffix, because any longer suffix takes
    precedence over the empty string.

Exploiting this property we can easily generalize suffix reordering
and specify the order of tags with given suffixes not only before but
even after a main release tag by using the empty suffix to denote the
position of the main release tag, without any algorithm changes:

  $ git -c versionsort.prereleaseSuffix=-alpha \
        -c versionsort.prereleaseSuffix=-beta \
        -c versionsort.prereleaseSuffix="" \
        -c versionsort.prereleaseSuffix=-gamma \
        -c versionsort.prereleaseSuffix=-delta \
        tag -l --sort=version:refname 'v3.0*'
  v3.0-alpha1
  v3.0-beta1
  v3.0
  v3.0-gamma1
  v3.0-delta1

Since 'versionsort.prereleaseSuffix' is not a fitting name for a
configuration variable to control this more general suffix reordering,
introduce the new variable 'versionsort.suffix'.  Still keep the old
configuration variable name as a deprecated alias, though, to avoid
suddenly breaking setups already using it.  Ignore the old variable if
both old and new configuration variables are set, but emit a warning
so users will be aware of it and can fix their configuration.  Extend
the documentation to describe and add a test to check this more
general behavior.

Note: since the empty suffix matches all tagnames, tagnames with
suffixes not included in the configuration are listed together with
the suffixless main release tag, ordered lexicographically right after
that, i.e. before tags with suffixes listed in the configuration
following the empty suffix.

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