git-gui: fix incorrect use of Tcl append command

Fix wrong use of append command in strings marked for translation.
According to Tcl/Tk Documentation [1],
	append varName ?value value value ...?
appends all value arguments to the current value of variable varName.
This means that
	append "[appname] ([reponame]): " [mc "File Viewer"]
is setting a variable named "[appname] ([reponame]): " to the output of
[mc "File Viewer"], rather than returning the concatenation of both
expressions as one might expect.

The format for some strings enables, for instance, a French translator
to translate like "%s (%s) : Create Branch" (space before colon).
Conversely, strings already translated will be marked as fuzzy and the
translator must update them herself.

For some cases, use alternative way for concatenation instead of using
strcat procedure defined in git-gui.sh.

Reference: 31bb1d1 ("git-gui: Paper bag fix missing translated strings",
2007-09-14) fixes the same issue slightly differently.

[1] http://www.tcl.tk/man/tcl/TclCmd/append.htm

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

lib/error.tcl

@@ -17,7 +17,7 @@ proc error_popup {msg} {
 	set cmd [list tk_messageBox \
 		-icon error \
 		-type ok \
-		-title [append "$title: " [mc "error"]] \
+		-title [mc "%s: error" $title] \
 		-message $msg]
 	if {[winfo ismapped [_error_parent]]} {
 		lappend cmd -parent [_error_parent]
@@ -33,7 +33,7 @@ proc warn_popup {msg} {
 	set cmd [list tk_messageBox \
 		-icon warning \
 		-type ok \
-		-title [append "$title: " [mc "warning"]] \
+		-title [mc "%s: warning" $title] \
 		-message $msg]
 	if {[winfo ismapped [_error_parent]]} {
 		lappend cmd -parent [_error_parent]