This package can be build with git-buildpackage.
The source tarball can be downloaded with uscan.

So building the package is:
1. Clone the repo
  gbp clone https://salsa.debian.org/matthias-guest/pkg-munin-c.git

2. Change to the fresh cloned directory
  cd pkg-munin-c

3. Build with gbp
  gbp buildpackage

Upgrade to a new version:
1. Check if a new version is available:
  uscan --report --verbose

2. Download new upstream tarball if there is a new version (e.g. 0.0.13)
  uscan --download --verbose

3. Import the tarball to the git repository
  gbp import-orig ../munin-c_0.0.13.orig.tar.gz

