Merge branch 'jn/ident-from-etc-mailname'

* jn/ident-from-etc-mailname:
  ident: do not retrieve default ident when unnecessary
  ident: check /etc/mailname if email is unknown

Documentation/git-commit-tree.txt

@@ -68,7 +68,9 @@ if set:
 
 In case (some of) these environment variables are not set, the information
 is taken from the configuration items user.name and user.email, or, if not
-present, system user name and fully qualified hostname.
+present, system user name and the hostname used for outgoing mail (taken
+from `/etc/mailname` and falling back to the fully qualified hostname when
+that file does not exist).
 
 A commit comment is read from stdin. If a changelog
 entry is not provided via "<" redirection, 'git commit-tree' will just wait
@@ -90,6 +92,10 @@ Discussion
 
 include::i18n.txt[]
 
+FILES
+-----
+/etc/mailname
+
 SEE ALSO
 --------
 linkgit:git-write-tree[1]