Merge branch 'jk/mailmap-only-at-root'

The .mailmap is documented to be read only from the root level of a
working tree, but a stray file in a bare repository also was read
by accident, which has been corrected.

* jk/mailmap-only-at-root:
  mailmap: only look for .mailmap in work tree

Documentation/git-shortlog.txt

@@ -113,6 +113,10 @@ MAPPING AUTHORS
 
 See linkgit:gitmailmap[5].
 
+Note that if `git shortlog` is run outside of a repository (to process
+log contents on standard input), it will look for a `.mailmap` file in
+the current directory.
+
 GIT
 ---
 Part of the linkgit:git[1] suite