gitk: prevent overly long command lines

To avoid running into command line limitations, some of Git's commands support the `--stdin` option. Let's use exactly this option in the three rev-list/log invocations in gitk that would otherwise possibly run the danger of trying to invoke a too-long command line. While it is easy to redirect either stdin or stdout in Tcl/Tk scripts, what we need here is both. We need to capture the output, yet we also need to pipe in the revs/files arguments via stdin (because stdin does not have any limit, unlike the command line). To help this, we use the neat Tcl feature where you can capture stdout and at the same time feed a fixed string as stdin to the spawned process. One non-obvious aspect about this change is that the `--stdin` option allows to specify revs, the double-dash, and files, but *no* other options such as `--not`. This is addressed by prefixing the "negative" revs with `^` explicitly rather than relying on the `--not` option (thanks for coming up with that idea, Max!). This fixes https://github.com/git-for-windows/git/issues/1987 Analysis-and-initial-patch-by: Max Kirillov <max@max630.net> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>