diffcore-pickaxe: make error messages more consistent

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>