various issues with filtering commits from log

[Oswald Buddenhagen <oswald.buddenhagen@gmx.de>]

moin!

i have to deal with a repo which has relatively little traffic, except 
that a bot is constantly spamming it, making it basically unfeasible to 
get a quick overview of what happened.

so i thought, "there certainly is an --exclude-author option, and 
ideally, i can persist it in the config as the default".
well, no. what i got is "--perl-regexp --author='^(?!The Bot)'". that's 
.. not user friendly.

once i got there, i noticed that --graph tears apart the history. the 
manual talks about parent rewriting, but clearly this is not applied to 
_this_ kind of history simplification.

so next i tried to approximate the desired result by logging only the 
most interesting subdirectory, which the bot luckily doesn't touch. this 
actually looks kinda reasonable, with the catch that a lot of tags & 
branches are missing from the decorations. this of course makes sense, 
as the relevant commits are not listed. however, it would be rather 
desirable to have parent rewriting be applied here as well, so "fake" 
decorations would be shown (with an indication where this was done).

these issues affect "log --all" particularly strongly, as it's not 
really clear what belongs where.

somewhat unsurprisingly, these issues also affect gitk.