git-gui: corrected setup of git worktree under cygwin.

Under cygwin the _gitworktree variable needs to contain the Windows
style path string so the output provided by git rev-parse must
be converted from cygwin path style to native.

Reviewed-by: Jesse Welch <jesse.welch@baml.com>
Signed-off-by: John Patrick Murphy <john.murphy@baml.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -1284,7 +1284,11 @@ apply_config
 
 # v1.7.0 introduced --show-toplevel to return the canonical work-tree
 if {[package vsatisfies $_git_version 1.7.0]} {
-	set _gitworktree [git rev-parse --show-toplevel]
+	if { [is_Cygwin] } {
+		catch {set _gitworktree [exec cygpath --windows [git rev-parse --show-toplevel]]}
+	} else {
+		set _gitworktree [git rev-parse --show-toplevel]
+	}
 } else {
 	# try to set work tree from environment, core.worktree or use
 	# cdup to obtain a relative path to the top of the worktree. If