gitk: Handle msysGit version during version comparisons

msysGit generates version strings with text appended which cannot
be used with vcompare; trying to use them generates a Tcl error.
Limit git_version to the first three digits which are the real git
version to avoid this error.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -11152,7 +11152,7 @@ set nullid2 "0000000000000000000000000000000000000001"
 set nullfile "/dev/null"
 
 set have_tk85 [expr {[package vcompare $tk_version "8.5"] >= 0}]
-set git_version [lindex [exec git version] end]
+set git_version [join [lrange [split [lindex [exec git version] end] .] 0 2] .]
 
 set runq {}
 set history {}