git-gui: warn when trying to commit on a detached head

The commandline is already warning when checking out a detached head.
Since the only thing thats potentially dangerous is to create commits
on a detached head lets warn in case the user is about to do that.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
diff --git a/lib/commit.tcl b/lib/commit.tcl
index 5ce4687..372bed9 100644
--- a/lib/commit.tcl
+++ b/lib/commit.tcl
@@ -260,8 +260,23 @@
 }
 
 proc commit_commitmsg {curHEAD msg_p} {
+	global is_detached repo_config
 	global pch_error
 
+	if {$is_detached && $repo_config(gui.warndetachedcommit)} {
+		set msg [mc "You are about to commit on a detached head.\
+This is a potentially dangerous thing to do because if you switch\
+to another branch you will loose your changes and it can be difficult\
+to retrieve them later from the reflog. You should probably cancel this\
+commit and create a new branch to continue.\n\
+\n\
+Do you really want to proceed with your Commit?"]
+		if {[ask_popup $msg] ne yes} {
+			unlock_index
+			return
+		}
+	}
+
 	# -- Run the commit-msg hook.
 	#
 	set fd_ph [githook_read commit-msg $msg_p]