repo_update is currently not reentrant: If a commit is picked once it
cannot be picked a second time as that either results in no changes or a
bunch of merge conflicts.

Generate stable hashes with patch-id that are invariant to line number
changes, white space and file ordering, and use those to compare with
existing patches on top of the selected manifest tag. Skip the pick on
matching hashes.

Note that this intentionally does not check if the patch is found
anywhere else in the tree: It might have been picked and reverted
multiple times in AOSP. Furthermore, if the patch eventually ends up on
a tag we want to be alerted too and remove it.
This obviously implies that we should not be applying and reverting our
own patches either.

Signed-off-by: Marijn Suijten <[email protected]>

This variant operates on reverts instead of cherry picks. See the
previous commit for an overview of the method and design choices.

Reverts are compared by creating a patch that is the revert of the
desired commit, of which the stable patch-id hash is used to compare
against the commit range. Notable is the mnemonic prefix (a/ and b/)
which are swapped in that revert as well, resulting in mismatched
hashes. This is addressed by showing the patch instead with
diff.noprefix=true.

Signed-off-by: Marijn Suijten <[email protected]>