Revert "git-gui: remove lines starting with the comment character"

Revert "git-gui: remove lines starting with the comment character"

This reverts commit b9a43869c9f96d3577d6f568c1bda1940c8f0e31.

This commit causes breakage on macOS (10.13). It causes errors on
startup and completely breaks the commit functionality. There are two
main problems. First, it uses `string cat` which is not supported on
older Tcl versions. Second, it does a half close of the bidirectional
pipe to git-stripspace which is also not supported on older Tcl
versions.

Reported-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>