Don't track new files in submodules from main repo

Prevent main repo `git status` reporting submodules having new files.
New files are usually zcompiled zsh files.
There's no way to .gitignore from main repo specific files in submodules

Closes zimfw#297

Author: kaznovac

.gitmodules

@@ -1,21 +1,28 @@
 [submodule "modules/history-substring-search/external"]
 	path = modules/history-substring-search/external
 	url = https://github.com/zsh-users/zsh-history-substring-search.git
+	ignore = untracked
 [submodule "modules/completion/external"]
 	path = modules/completion/external
 	url = https://github.com/zsh-users/zsh-completions.git
+	ignore = untracked
 [submodule "modules/syntax-highlighting/external"]
 	path = modules/syntax-highlighting/external
 	url = https://github.com/zsh-users/zsh-syntax-highlighting.git
+	ignore = untracked
 [submodule "modules/prompt/external-themes/pure"]
 	path = modules/prompt/external-themes/pure
 	url = https://github.com/sindresorhus/pure.git
+	ignore = untracked
 [submodule "modules/prompt/external-themes/liquidprompt"]
 	path = modules/prompt/external-themes/liquidprompt
 	url = https://github.com/nojhan/liquidprompt.git
+	ignore = untracked
 [submodule "modules/prompt/external-themes/lean"]
 	path = modules/prompt/external-themes/lean
 	url = https://github.com/miekg/lean
+	ignore = untracked
 [submodule "modules/autosuggestions/external"]
 	path = modules/autosuggestions/external
 	url = https://github.com/zsh-users/zsh-autosuggestions.git
+	ignore = untracked