Travis clang-format, cpplint: Use the merge base

[tautschnig]

We previously looked at the diff to the current head of the target
branch, which may have moved in comparison to the merge base of a pull
request. Thus we sometimes ended up with spurious clang-format errors,
talking about changes introduced in another PR.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[kroening]

That certainly did happen to me.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 181a74d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113876156
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 55a8272).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113879798

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 3280c41).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113889235

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@24212bf). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #4740   +/-   ##
==========================================
  Coverage           ?   68.44%           
==========================================
  Files              ?     1255           
  Lines              ?   104094           
  Branches           ?        0           
==========================================
  Hits               ?    71250           
  Misses             ?    32844           
  Partials           ?        0

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 24212bf...3280c41. Read the comment docs.

[smowton]

Double check: how does this behave on a push build?

[tautschnig]

Double check: how does this behave on a push build?

Travis' documentation says: "The range of commits that were included in the push or pull request. (Note that this is empty for builds triggered by the initial commit of a new branch.)" So I'd expect that to work exactly the same way in both cases.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 59baaab).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113954417
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.