git-gui: allow opening currently selected file in default app

Many times there's the need to quickly open a source file (the one you're
looking at in Git GUI) in the predefined text editor / IDE. Of course,
the file can be searched for in your preferred file manager or directly
in the text editor, but having the option to directly open the current
file from Git GUI would be just faster. This change enables just that by:
 - clicking the diff header path (which is now highlighted as a hyperlink)
 - or diff header path context menu -> Open

Note: executable files will be run and not opened for editing.

Signed-off-by: Zoli Szabó <zoli.szabo@gmail.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

git-gui.sh

@@ -2248,9 +2248,8 @@ proc do_git_gui {} {
 	}
 }
 
-proc do_explore {} {
-	global _gitworktree
-	set explorer {}
+# Get the system-specific explorer app/command.
+proc get_explorer {} {
 	if {[is_Cygwin] || [is_Windows]} {
 		set explorer "explorer.exe"
 	} elseif {[is_MacOSX]} {
@@ -2259,9 +2258,23 @@ proc do_explore {} {
 		# freedesktop.org-conforming system is our best shot
 		set explorer "xdg-open"
 	}
+	return $explorer
+}
+
+proc do_explore {} {
+	global _gitworktree
+	set explorer [get_explorer]
 	eval exec $explorer [list [file nativename $_gitworktree]] &
 }
 
+# Open file relative to the working tree by the default associated app.
+proc do_file_open {file} {
+	global _gitworktree
+	set explorer [get_explorer]
+	set full_file_path [file join $_gitworktree $file]
+	exec $explorer [file nativename $full_file_path] &
+}
+
 set is_quitting 0
 set ret_code    1
 
@@ -3513,9 +3526,11 @@ tlabel .vpane.lower.diff.header.file \
 	-justify left
 tlabel .vpane.lower.diff.header.path \
 	-background gold \
-	-foreground black \
+	-foreground blue \
 	-anchor w \
-	-justify left
+	-justify left \
+	-font [eval font create [font configure font_ui] -underline 1] \
+	-cursor hand2
 pack .vpane.lower.diff.header.status -side left
 pack .vpane.lower.diff.header.file -side left
 pack .vpane.lower.diff.header.path -fill x
@@ -3530,8 +3545,12 @@ $ctxm add command \
 			-type STRING \
 			-- $current_diff_path
 	}
+$ctxm add command \
+	-label [mc Open] \
+	-command {do_file_open $current_diff_path}
 lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
 bind_button3 .vpane.lower.diff.header.path "tk_popup $ctxm %X %Y"
+bind .vpane.lower.diff.header.path <Button-1> {do_file_open $current_diff_path}
 
 # -- Diff Body
 #