RFR: pr: improve message for unexpected merge commits [v2]

[Erik Helin]

> Hi all,
> 
> please review this patch that improves the warning when a pull request contains a merge request that brings in commits
> not present in the target repository. This indicates that the user might wanted to do a "merge style" pull request, but
> can also arise in other scenarios.  Testing:
> - [x] `make test` passes on Linux x64
> 
> Thanks,
> Erik

Erik Helin has updated the pull request incrementally with one additional commit since the last revision:

  Fix grammar
  
  Co-authored-by: Robin Westberg <robin at westberg.se>

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/693/files
  - new: https://git.openjdk.java.net/skara/pull/693/files/a4b94acb..9a40718a

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/693/webrev.01
 - incr: https://webrevs.openjdk.java.net/skara/693/webrev.00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/693.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/693/head:pull/693

PR: https://git.openjdk.java.net/skara/pull/693