gitk: work around the command line limit on Windows

On Windows, there are dramatic problems when a command line grows
beyond PATH_MAX, which is restricted to 8191 characters on XP and
later (according to http://support.microsoft.com/kb/830473).

Work around this by just cutting off the command line at that length
(actually, at a space boundary) in the hope that only negative
refs are chucked: gitk will then do unnecessary work, but that is
still better than flashing the gitk window and exiting with exit
status 5 (which no Windows user is able to make sense of).

The first fix caused Tcl to fail to compile the regexp, see msysGit issue
427. Here is another fix without using regexp, and using a more relaxed
command line length limit to fix the original issue 387.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

gitk-git/gitk

@@ -10172,7 +10172,19 @@ proc getallcommits {} {
 	}
     }
     if {$ids ne {}} {
-	set fd [open [concat $cmd $ids] r]
+	set cmd [concat $cmd $ids]
+	# The maximum command line length for the CreateProcess function is 32767 characters, see
+	# http://blogs.msdn.com/oldnewthing/archive/2003/12/10/56028.aspx
+	# Be a little conservative in case Tcl adds some more stuff to the command line we do not
+	# know about and truncate the command line at a SHA1-boundary below 32000 characters.
+	if {[tk windowingsystem] == "win32" &&
+		[string length $cmd] > 32000} {
+	    set ndx [string last " " $cmd 32000]
+	    if {$ndx != -1} {
+		set cmd [string range $cmd 0 $ndx]
+	    }
+	}
+	set fd [open $cmd r]
 	fconfigure $fd -blocking 0
 	incr allcommits
 	nowbusy allcommits