Add version and gitcommit to the build ldflags. (Closes: #909644)

We need to maintain a table to know what is the git commit for a given
tag. This table is in the file `debian/upstream-version-gitcommit`.

This commit also adds a simple helper to retrieve the git commit easily
from the command-line, given the version. Just so that it's easy to
maintain this table in the future.

Signed-off-by: Arnaud Rebillout <arnaud.rebillout@collabora.com>