gitk: Add "--no-replace-objects" option

Replace refs are useful to change some git objects after they
have started to be shared between different repositories. One
might want to ignore them to see the original state, and
"--no-replace-objects" option can be used from the command
line to do so.

This option simply sets the GIT_NO_REPLACE_OBJECTS environment
variable, and that is enough to make gitk ignore replace refs.

The GIT_NO_REPLACE_OBJECTS is set to "1" instead of "" as it is
safer on some platforms, thanks to Johannes Sixt and Michael J
Gruber.

Tested-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -130,7 +130,7 @@ proc unmerged_files {files} {
 }
 
 proc parseviewargs {n arglist} {
-    global vdatemode vmergeonly vflags vdflags vrevs vfiltered vorigargs
+    global vdatemode vmergeonly vflags vdflags vrevs vfiltered vorigargs env
 
     set vdatemode($n) 0
     set vmergeonly($n) 0
@@ -210,6 +210,9 @@ proc parseviewargs {n arglist} {
 		# git rev-parse doesn't understand --merge
 		lappend revargs --gitk-symmetric-diff-marker MERGE_HEAD...HEAD
 	    }
+	    "--no-replace-objects" {
+		set env(GIT_NO_REPLACE_OBJECTS) "1"
+	    }
 	    "-*" {
 		# Other flag arguments including -<n>
 		if {[string is digit -strict [string range $arg 1 end]]} {