version: fix builtin linking & documentation

version: fix builtin linking & documentation

Like most builtins, 'version' is documented in a corresponding
'Documentation/git-version.txt' and can be invoked with 'git version'.
However, the 'check-docs' Makefile target showed that it was "removed but
documented: git-version." This was cause by the fact that it is not built as
a standalone 'git-version' executable, therefore appearing "removed" to
'check-docs'.

Without a precedent for documented builtins that aren't built into an
executable *or* any clear reason why a standalone 'git-version' shouldn't
exist, the 'check-docs' error appears to correctly identify an issue. To
correct that mismatch, add 'git-version' to the 'BUILT_INS' list in the root
Makefile (indicating that the 'cmd_version()' function appears in a file
that is *not* 'builtin/version.c'). Additionally, to avoid the "no link"
message in 'check-docs', list 'git-version' as an "ancilliaryinterrogator"
(like 'git help') in 'command-list.txt'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>