Allow git-cvsserver database table name prefix to be specified.

Allow git-cvsserver database table name prefix to be specified.

Adds a gitcvs.dbtablenameprefix config variable, the contents of which
are prepended to any database tables names used by git-cvsserver. The
same substutions as gitcvs.dbname and gitcvs.dbuser are supported, and
any non-alphabetic characters are replaced with underscores.

A typo found in contrib/completion/git-completion.bash is also fixed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>