The golang-gopkg-src-d-go-git.v4 package is dead since 6 years ago:

https://github.com/src-d/go-git

Replaced by https://github.com/go-git/go-git which is already in Debian.

The only reverse dependency in Debian is 'mgmt' but it already has a
FTBFS bug and the B-D dependency is obsolete: the go.mod file already
indicate upstream moved to the new package and that package is part of
debian/control Build-Depends.  Removing golang-gopkg-src-d-go-git.v4
won't make things worse in unstable, but will make things better since
we don't have to worry about that obsolete package any more.

/Simon

jas@frallan:~/dpkg$ ssh mirror.ftp-master.debian.org dak rm -Rn golang-gopkg-src-d-go-git.v4
Will remove the following packages from unstable:

golang-gopkg-src-d-go-git.v4 |   4.11.0-3 | source
golang-gopkg-src-d-go-git.v4-dev |   4.11.0-3 | all

Maintainer: Debian Go Packaging Team <team+pkg-go@tracker.debian.org>
------------------- Reason -------------------

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

Checking reverse dependencies...
# Broken Build-Depends:
mgmt: golang-gopkg-src-d-go-git.v4-dev

Dependency problem found.

jas@frallan:~/dpkg$

We believe that the bug you reported is now fixed; the following
package(s) have been removed from unstable:

golang-gopkg-src-d-go-git.v4 |   4.11.0-3 | source
golang-gopkg-src-d-go-git.v4-dev |   4.11.0-3 | all

------------------- Reason -------------------
ROTeamM; unmaintained and replaced
----------------------------------------------

Note that the package(s) have simply been removed from the tag
database and may (or may not) still be in the pool; this is not a bug.
The package(s) will be physically removed automatically when no suite
references them (and in the case of source, when no binary references
it).  Please also remember that the changes have been done on the
master archive and will not propagate to any mirrors until the next
dinstall run at the earliest.

Packages are usually not removed from testing by hand. Testing tracks
unstable and will automatically remove packages which were removed
from unstable when removing them from testing causes no dependency
problems. The release team can force a removal from testing if it is
really needed, please contact them if this should be the case.

We try to close bugs which have been reported against this package
automatically. But please check all old bugs, if they were closed
correctly or should have been re-assigned to another package.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1144140@bugs.debian.org.

The full log for this bug can be viewed at https://bugs.debian.org/1144140

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

Debian distribution maintenance software
pp.
Thorsten Alteholz (the ftpmaster behind the curtain)