Add .gitconfig enabling diff'ing for encrypted .age files

.gitattributes

@@ -7,4 +7,5 @@ dot_profile linguist-language=Shell
 dot_gitconfig linguist-language=TOML linguist-detectable
 *.toml linguist-detectable
 *.toml.tmpl linguist-language=TOML linguist-detectable
-*.cfg.tmpl linguist-language=INI linguist-detectable
+*.cfg.tmpl linguist-language=INI linguist-detectable
+*.age diff=age

.gitconfig

@@ -0,0 +1,2 @@
+[diff "age"]
+  textconv = "age -d -i ~/key.txt"

home/run_onchange_gitconfig.sh.tmpl

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# We just need to have access to the Git repository
+cd {{ .chezmoi.sourceDir }}
+# The path is relative to the .git/config file
+git config --local include.path ../.gitconfig