hg-{fast-export,reset}.sh: add git --exec-path to PATH

hg-{fast-export,reset}.sh: add git --exec-path to PATH

Starting with Git 2.10.0, the git-sh-setup script expects
the git --exec-path directory to be in the PATH or it will
fail.  Although there is a patch in the works to correct
this, many of the other scripts in the --exec-path directory
also assume the same thing.

Explicitly add git --exec-path to the front of the PATH just
like Git normally does so that git-sh-setup starts working
again without need of the patch and to future-proof the code
against similar breakage in the future.

As a result also switch to just plain ". git-sh-setup" to match
how the rest of the scripts in --exec-path run the setup script.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>