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>

git-gui.sh

@@ -854,6 +854,7 @@ set default_config(gui.fontdiff) [font configure font_diff]
 # TODO: this option should be added to the git-config documentation
 set default_config(gui.maxfilesdisplayed) 5000
 set default_config(gui.usettk) 1
+set default_config(gui.warndetachedcommit) 1
 set font_descs {
 	{fontui   font_ui   {mc "Main Font"}}
 	{fontdiff font_diff {mc "Diff/Console Font"}}

lib/commit.tcl

@@ -260,8 +260,23 @@ proc commit_prehook_wait {fd_ph curHEAD msg_p} {
 }
 
 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]