I noticed in the static deltas tests, there were some tests that
should have been under `-o pipefail` to ensure we properly propagate
errors.
There were a few places where we were referencing undefined variables.
Overall, this is clearly a good idea IMO.
Use the parse-datetime module from gnulib, and adapt it to not require
other modules as portability is not really an issue for us.
DATE can be specified in different formats, such as: "-1 week", "last
monday", "1 week ago".
Include the generated .c file in the repository so to not add another
dependency to Bison.
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>