gc.sh: use --git-dir= instead of GIT_DIR=

gc.sh: use --git-dir= instead of GIT_DIR=

Using GIT_DIR= has an unfortunate interaction with the git() wrapper
in shlib.sh that can leave GIT_DIR set when it was supposed to be
set only temporarily.

Therefore switch from using GIT_DIR= to --git-dir= instead to avoid
this problem.

Fortunately in this case there were no ill effects because thanks to
the "get_repo_list | while read" pipe the problematic GIT_DIR= was
running in a subshell and could not adversely affect any other code.

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