git: overhaul git version checking

git: overhaul git version checking

Rework the git version tetsting to handle versions like "1.5.2-rc3"
or (apparently) "1.5.2-rc3 (something ...)".  Add tests for parsing of
all the version types in the current git tag history that we need to
support.

Support and document BUP_ASSUME_GIT_VERSION_IS_FINE=1 as an escape
hatch in case the parsing isn't sufficiently comprehensive, or
upstreeam changes their practices in future releases.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>