When building using the Branch Source plugin setting so that PR's are discovered as merged with master the commit hash put in the env GIT_COMMIT is not a correct value.

 
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Declarative: Checkout SCM)
[Pipeline] checkout
Fetching changes from the remote Git repository
Cleaning workspace
Fetching without tags
Merging remotes/origin/master commit 327f14dfdcdf41c01089767ae1ee35409df56d0e into PR head commit b03e30ec40954b8b40ca85374e4cf1d8eb3511cb
Merge succeeded, producing 7461047c83a98c6ded075bba1bc44f2386d526d6
Checking out Revision 7461047c83a98c6ded075bba1bc44f2386d526d6 (PR-12)
{{ > git rev-parse --is-inside-work-tree # timeout=10}}
{{ > git config remote.origin.url ... # timeout=10}}
{{ > git rev-parse --verify HEAD # timeout=10}}
Resetting working tree
{{ > git reset --hard # timeout=10}}
{{ > git clean -fdx # timeout=10}}
Fetching upstream changes from ...
{{ > git --version # timeout=10}}
using GIT_SSH to set credentials CI användare i GitHub
{{ > git fetch --no-tags --progress ... +refs/pull/12/head:refs/remotes/origin/PR-12 +refs/heads/master:refs/remotes/origin/master}}
{{ > git config core.sparsecheckout # timeout=10}}
{{ > git checkout -f b03e30ec40954b8b40ca85374e4cf1d8eb3511cb}}
{{ > git merge 327f14dfdcdf41c01089767ae1ee35409df56d0e # timeout=10}}
{{ > git rev-parse HEAD^

# timeout=10}}
{{ > git config core.sparsecheckout # timeout=10}}
{{ > git checkout -f 7461047c83a98c6ded075bba1bc44f2386d526d6}}
Commit message: "Merge commit '327f14dfdcdf41c01089767ae1ee35409df56d0e' into HEAD"
First time build. Skipping changelog.
Cleaning workspace

...

(later when I print $GIT_COMMIT in an sh step)
GIT_COMMIT=26053fb12db6deb1f9b129b3d6dbeae505cbae39

...

(to provide som debug info I issue some git command in the same sh step
latest commit seams ok, since its the result of Jenkins merge locally in the workspace)
08:42:44 + git log -10
08:42:44 commit 7461047c83a98c6ded075bba1bc44f2386d526d6
08:42:44 Merge: b03e30e 327f14d
08:42:44 Author: Jenkins <nobody@nowhere>
08:42:44 Date: Tue Jan 15 08:42:40 2019 +0100
{{08:42:44 }}
08:42:44 Merge commit '327f14dfdcdf41c01089767ae1ee35409df56d0e' into HEAD
08:42:44

...

(git show $GIT_COMMIT fails as the hash do not exist)
08:42:44 + git show 26053fb12db6deb1f9b129b3d6dbeae505cbae39
08:42:44 fatal: bad object 26053fb12db6deb1f9b129b3d6dbeae505cbae39 

One maybe related issue is that we are depending on an shared library and I see that there has been changes to the behavior around shared libraries.

This worked on:

Git Plugin 3.8.0
Branch Source Plugin 2.3.3
Jenkins 2.138.3