git-gui: Detect full path when parsing arguments

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>