diffcore-pickaxe: make error messages more consistent
Currently, diffcore-pickaxe reports two distinct errors for the same
user error:
$ git log --pickaxe-regex -S'\1'
fatal: invalid pickaxe regex: Invalid back reference
$ git log -G'\1'
fatal: invalid log-grep regex: Invalid back reference
This "log-grep" was only an internal name for the -G feature during
development, and invite confusion with "git log --grep=<pattern>".
Change the error messages to say "invalid regex".
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>