git-gui: Don't prepend the prefix if value looks like a full path

When argument parsing fails to detect a file name, "git-gui" will try to
use the previously detected "head" as the file name. We should avoid
prepending the prefix if "head" looks like a full path.
Signed-off-by: NAndrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -3037,8 +3037,13 @@ blame {
 	unset is_path
 
 	if {$head ne {} && $path eq {}} {
-		set path [normalize_relpath $_prefix$head]
-		set head {}
+		if {[string index $head 0] eq {/}} {
+			set path [normalize_relpath $head]
+			set head {}
+		} else {
+			set path [normalize_relpath $_prefix$head]
+			set head {}
+		}
 	}
 
 	if {$head eq {}} {