RFR: git-pr: rename "update" to "set"

[Erik Helin]

Hi all,

please review this patch that renames the `git pr` subcommand `update` to `set`.
This is done for a couple of reasons:

- `update` can be viewed as appending or enhancing whereas `set` better reflects
  that we do set some fields (i.e. all existing values for the field has to be
  repeated for appending).
- `update` is freed up to be user for future commands
- `set` is shorter than `update` ��

Examples of how the command is intended to be used are `git pr set
--assignees=rwestberg`, `git pr set --reviewers=JornVernee` and `git pr set
--no-draft`.

Testing:
- Manual testing of `git pr set`

Thanks,
Erik

-------------

Commits:
 - 120a5741: git-pr: rename "update" to "set"

Changes: https://git.openjdk.java.net/skara/pull/449/files
 Webrev: https://webrevs.openjdk.java.net/skara/449/webrev.00
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/449.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/449/head:pull/449

PR: https://git.openjdk.java.net/skara/pull/449