flake8: Increase max line length from 80 to 100

The Google style guide for python [1] says the maximum line length
should be 80, but there are several lines in the code base that
exceed it:

  git ls-files | grep py$ | xargs flake8 | grep E501 | wc -l
  64

I don't think it's worth going through and re-wrapping all those,
so just increase the limit to 100 which seems to be a reasonable
compromise:

  git ls-files | grep py$ | xargs flake8 | grep E501 | wc -l
  6

Leave the re-rewrapping of those lines for a follow-up commit,
though.

[1] http://google.github.io/styleguide/pyguide.html#32-line-length

Change-Id: Ia37c34301163431fd1fb4fb6697a4a482d6be077
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254595
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>