Teach the --multiple option to 'git fetch'

Add the --multiple option to specify that all arguments are either
groups or remotes. The primary reason for adding this option is
to allow us to re-implement 'git remote update' using fetch.

It would have been nice if this option was not needed, but since
the colon in a refspec is optional, it is in general not possible
to know whether a single, colon-less argument is a remote or a
refspec.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5514-fetch-multiple.sh

@@ -73,4 +73,48 @@ test_expect_success 'git fetch --all does not allow non-option arguments' '
 	 test_must_fail git fetch --all origin master)
 '
 
+cat > expect << EOF
+  origin/HEAD -> origin/master
+  origin/master
+  origin/side
+  three/another
+  three/master
+  three/side
+EOF
+
+test_expect_success 'git fetch --multiple (but only one remote)' '
+	(git clone one test3 &&
+	 cd test3 &&
+	 git remote add three ../three &&
+	 git fetch --multiple three &&
+	 git branch -r > output &&
+	 test_cmp ../expect output)
+'
+
+cat > expect << EOF
+  one/master
+  one/side
+  origin/HEAD -> origin/master
+  origin/master
+  origin/side
+  two/another
+  two/master
+  two/side
+EOF
+
+test_expect_success 'git fetch --multiple (two remotes)' '
+	(git clone one test4 &&
+	 cd test4 &&
+	 git remote add one ../one &&
+	 git remote add two ../two &&
+	 git fetch --multiple one two &&
+	 git branch -r > output &&
+	 test_cmp ../expect output)
+'
+
+test_expect_success 'git fetch --multiple (bad remote names)' '
+	(cd test4 &&
+	 test_must_fail git fetch --multiple four)
+'
+
 test_done