Move code for git info to its own shell script "version-ext.sh".
* New version-ext.sh file: sh script to output MPFR version information for a Git working tree (old code from tests/Makefile.am). * tests/Makefile.am: replace some output_info code by the use of this script. * Makefile.am: add version-ext.sh to EXTRA_DIST as a consequence (a comment explains why). * README: list this new version-ext.sh file as a consequence.