Adding optionally commit number in PG_VERSION_STR

Hi all,

It happens that I work occasionally on multiple builds based on
different stable branches at the same time to check fixes that need to
be backpatched, and I tend to easily lose track on which version the
build I created is based on (Duh!). There is of course the version
number up to the 3rd digit available (for example 9.2.4, 9.3beta2,
etc.), but as a developer I think that it would be helpful to include
the commit ID in PG_VERSION_STR to get a better reference on exactly
what the development build is based on. This could be controlled by an
additional flag in ./configure.in called something like
--enable-version-commit, of course disabled by default. If enabled,
PG_VERSION_STR would be generated with the new information. configure
would also return an error when this flag is enabled if git is either
not found, or if the repository where configure is not a native git
repository.

Thoughts?
--
Michael