Read user.name and email from environment first

Read user.name and email from environment first

According to [1] user name and email are taken first from the
environment variables:
GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL
GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL

In case (some of) these environment variables are not set, the
information is taken from the git configuration.

JGit doesn not yet support the environment variables GIT_AUTHOR_DATE and
GIT_COMMITTER_DATE.

[1] https://www.kernel.org/pub/software/scm/git/docs/git-commit-tree.html#_commit_information

Bug: 460586
Change-Id: I3ba582b4ae13674cf319652b5b13ebcbb96dd8ec
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>