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' # --pickaxe-regex is implied
    fatal: invalid log-grep regex: Invalid back reference

Since the error has nothing to do with "log-grep", change the -G error
message to match the -S error message.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
diffcore-pickaxe doc: document -S and -G properly

The documentation of -S and -G is very sketchy.  Completely rewrite the
sections in Documentation/diff-options.txt and
Documentation/gitdiffcore.txt.

References:
52e9578 ([PATCH] Introducing software archaeologist's tool "pickaxe".)
f506b8e (git log/diff: add -G<regexp> that greps in the patch text)

Inputs-from: Phil Hord <phil.hord@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>