diff --git a/git-gui/lib/commit.tcl b/git-gui/lib/commit.tcl
index 11379f8ad355e2..208dc2817ca68c 100644
--- a/git-gui/lib/commit.tcl
+++ b/git-gui/lib/commit.tcl
@@ -207,8 +207,17 @@ You must stage at least 1 file before you can commit.
 
 	# -- A message is required.
 	#
-	set msg [string trim [$ui_comm get 1.0 end]]
+	set msg [$ui_comm get 1.0 end]
+	# Strip trailing whitespace
 	regsub -all -line {[ \t\r]+$} $msg {} msg
+	# Strip comment lines
+	regsub -all {(^|\n)#[^\n]*} $msg {\1} msg
+	# Strip leading empty lines
+	regsub {^\n*} $msg {} msg
+	# Compress consecutive empty lines
+	regsub -all {\n{3,}} $msg "\n\n" msg
+	# Strip trailing empty line
+	regsub {\n\n$} $msg "\n" msg
 	if {$msg eq {}} {
 		error_popup [mc "Please supply a commit message.
 
diff --git a/git-gui/lib/mergetool.tcl b/git-gui/lib/mergetool.tcl
index e688b016ef6c9d..8b8c16b1d616b6 100644
--- a/git-gui/lib/mergetool.tcl
+++ b/git-gui/lib/mergetool.tcl
@@ -272,8 +272,25 @@ proc merge_resolve_tool2 {} {
 		}
 	}
 	default {
-		error_popup [mc "Unsupported merge tool '%s'" $tool]
-		return
+		set tool_cmd [get_config mergetool.$tool.cmd]
+		if {$tool_cmd ne {}} {
+			if {([string first {[} $tool_cmd] != -1) || ([string first {]} $tool_cmd] != -1)} {
+				error_popup [mc "Unable to process square brackets in \"mergetool.%s.cmd\" configuration option.
+
+Please remove the square brackets." $tool]
+				return
+			} else {
+				set cmdline {}
+				foreach command_part $tool_cmd {
+					lappend cmdline [subst -nobackslashes -nocommands $command_part]
+				}
+			}
+		} else {
+			error_popup [mc "Unsupported merge tool '%s'.
+
+To use this tool, configure \"mergetool.%s.cmd\" as shown in the git-config manual page." $tool $tool]
+			return
+		}
 	}
 	}