Do not rely on git internals, support Git >= 2.10

Fast-export has traditionally sourced the internal git-sh-setup from
Git, following the release of Git 2.10 this no longer works. Fast-export
only uses the functionality of git-sh-setup for two things: cd:ing to
the git repo dir and setting up the GIT_REPO environment variable. To
future-proof fast-export start doing what we need by hand in
fast-export.

Acknowledgments to Louis Sautier who reported the problem and tested the
fix.

hg-fast-export.sh

@@ -48,8 +48,10 @@ case "$1" in
       echo "$LONG_USAGE"
       exit 0
 esac
-. "$(git --exec-path)/git-sh-setup"
-cd_to_toplevel
+
+cd $(git rev-parse --show-toplevel) \
+    || (echo "Could not find git repo" ; exit 1)
+GIT_DIR=$(git rev-parse --git-dir) || exit 1
 
 while case "$#" in 0) break ;; esac
 do

hg-reset.sh

@@ -24,8 +24,9 @@ Options:
 	-r	Mercurial repository to use
 "
 
-. "$(git --exec-path)/git-sh-setup"
-cd_to_toplevel
+cd $(git rev-parse --show-toplevel) \
+    || (echo "Could not find git repo" ; exit 1)
+GIT_DIR=$(git rev-parse --git-dir) || exit 1
 
 while case "$#" in 0) break ;; esac
 do