git-gui: Detect full path when parsing arguments

When running "git-gui blame" from a subfolder (which means prefix is
non-empty), if we pass a full path as argument, the argument parsing
will fail to recognize the argument as a file name, because prefix is
prepended to the argument.

This patch handles that scenario by adding an additional branch that
checks the file name without using the prefix.

Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -3003,10 +3003,19 @@ blame {
 	set jump_spec {}
 	set is_path 0
 	foreach a $argv {
-		if {$is_path || [file exists $_prefix$a]} {
+		if {[file exists $a]} {
+			if {$path ne {}} usage
+			set path [normalize_relpath $a]
+			break
+		} elseif {[file exists $_prefix$a]} {
 			if {$path ne {}} usage
 			set path [normalize_relpath $_prefix$a]
 			break
+		}
+
+		if {$is_path} {
+			if {$path ne {}} usage
+			break
 		} elseif {$a eq {--}} {
 			if {$path ne {}} {
 				if {$head ne {}} usage