mingit: query the current git.exe for the system config location

In git-for-windows/git#2358, we fixed the design
where Git for Windows would first read a "public" system config that was
not even in `C:\Program Files\Git` and then would read a "private"
system config. It now only reads _one_ system config, and that is in an
intuitive location: `C:\Program Files\Git\etc\gitconfig`.

Let's teach the script that generates the MinGit subset of Git for
Windows to make sure to ask `git.exe` what its idea of the system config
location is, and then use that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

mingit/release.sh

@@ -54,6 +54,11 @@ case "$SCRIPT_PATH" in
 	;;
 esac
 
+etc_gitconfig="$(git -c core.editor=echo config --system -e 2>/dev/null)" &&
+etc_gitconfig="$(cygpath -au "$etc_gitconfig")" &&
+etc_gitconfig="${etc_gitconfig#/}" ||
+die "Could not determine the path of the system config"
+
 rm -rf "$SCRIPT_PATH"/root &&
 mkdir -p "$SCRIPT_PATH"/root ||
 die "Could not create overlay directory"
@@ -72,7 +77,7 @@ test -z "$include_pdbs" || {
 die "Could not unpack .pdb files"
 
 # Make a list of files to include
-LIST="$(ARCH=$ARCH BITNESS=$BITNESS MINIMAL_GIT=1 \
+LIST="$(ARCH=$ARCH BITNESS=$BITNESS MINIMAL_GIT=1 ETC_GITCONFIG="$etc_gitconfig" \
 	PACKAGE_VERSIONS_FILE="$SCRIPT_PATH"/root/etc/package-versions.txt \
 	sh "$SCRIPT_PATH"/../make-file-list.sh "$@")" ||
 die "Could not generate file list"