Cirrus: Fix post-merge env. var. not set.

Cirrus-CI automatically sets `$CIRRUS_BASE_BRANCH` during PR testing.
This is used for the `build_each_commit` task, in order to compute the
commit-chain properly.  However, prior to this commit and after a PR
merges, the post-merge `build_each_commit` task would fail with
something similar to:

```
make build-all-new-commits GIT_BASE_BRANCH=origin/$CIRRUS_BASE_BRANCH |& ${TIMESTAMP}
[12:28:59] START - All [+xxxx] lines that follow are relative to right now.
[+0000s] # Validate that all the commits build on top of origin/
[+0000s] git rebase origin/ -x make
[+0000s] fatal: invalid upstream 'origin/'
[+0000s] make: *** [Makefile:426: build-all-new-commits] Error 128
[12:28:59] END - [+0000s] total duration since START
Exit status: 2
```

This is because `$CIRRUS_BASE_BRANCH` is undefined when CI runs against
a branch (by design).  This commit fixes the problem by referring to
`$DEST_BRANCH` instead.  This variable must always point at the intended
destination branch for testing, and so can be used in this context as
well.

Also updated a few comments to help steer understanding of the
`$DEST_BRANCH` purpose.

Signed-off-by: Chris Evich <cevich@redhat.com>

0 files changed, 0 insertions, 0 deletions