[Borgbackup] borg 1.2 pattern matching changes (compared to < 1.2)

[Thomas Waldmann]

long ago, we changed the pattern matching to be better aligned with how 
borg archives stuff: borg always archives relative paths.

e.g. filesystem /home/user/.bashrc will become home/user/.bashrc in the 
archive.

thus, we changed the pattern processing to apply to the paths you also 
see when you list a borg archive later. no leading slashes!

i am sorry that i overlooked this and forgot to include a prominent note 
about this change into the 1.2.0 releases' compatibility notes, we added 
this now (and also fixed the docs which where also not updated 
everywhere yet):

you can see the docs / examples changes in this diff:

https://github.com/borgbackup/borg/pull/6718/files

please understand that borg can not automatically deal with leading 
slashes in patterns (except in the most primitive cases), so you need to 
adapt them manually.

just do not try to match the leading slash, it is not there.

updated docs / changelog will be releases together with other fixes in 
1.2.1 later.