fix(git): Guard $gitConfig.name/email lookup against missing map keys

Accessing a missing key on a map value in text/template raises an
error before `default` can run, so `configs.git` without `name`/`email`
(or hosts without a `configs` key, e.g. termux-*) broke `chezmoi diff`
with "map has no entry for key". Use `hasKey` before accessing each
field instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0145M6KGu5gQ3brTPzqs8NwG

private_dot_config/git/config.tmpl

@@ -13,8 +13,14 @@
     {{- $gitConfig = $currentUser.configs.git -}}
   {{- end -}}
 {{- end -}}
-{{- $gitName := $gitConfig.name | default "Xavier Logerais" -}}
-{{- $gitEmail := $gitConfig.email | default "xavier@logerais.com" -}}
+{{- $gitName := "Xavier Logerais" -}}
+{{- $gitEmail := "xavier@logerais.com" -}}
+{{- if hasKey $gitConfig "name" -}}
+  {{- $gitName = $gitConfig.name -}}
+{{- end -}}
+{{- if hasKey $gitConfig "email" -}}
+  {{- $gitEmail = $gitConfig.email -}}
+{{- end -}}
 [core]
 	excludesfile = ~/.config/git/ignore
 	attributesfile = ~/.config/git/attributes