git-gui: Unconditionally use absolute paths with Cygwin

The mapping of Windows vs Cygwin/Posix paths is arbitrary and not knowable
from the Posix side, so logic to determine whether to use relative paths
to locate the git-gui library just does not work on Cygwin. Fix this to
unconditionally use absolute path on Cygwin. (This fixes a regression
from 1.5.4).

Signed-off-by: Mark Levedahl <mdl123@verizon.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Makefile

@@ -117,14 +117,12 @@ GITGUI_SCRIPT   := $$0
 GITGUI_RELATIVE :=
 GITGUI_MACOSXAPP :=
 
-ifeq ($(exedir),$(gg_libdir))
-	GITGUI_RELATIVE := 1
-endif
-
 ifeq ($(uname_O),Cygwin)
 	GITGUI_SCRIPT := `cygpath --windows --absolute "$(GITGUI_SCRIPT)"`
-	ifeq ($(GITGUI_RELATIVE),)
 	gg_libdir := $(shell cygpath --windows --absolute "$(gg_libdir)")
+else
+	ifeq ($(exedir),$(gg_libdir))
+		GITGUI_RELATIVE := 1
 	endif
 endif
 ifeq ($(uname_S),Darwin)