Force correct names for pip2/pip3 command in completions

Work around bug pypa/pip#4755
Don't expect all users to be running a version of pip2/3 that includes
the fix (once it's upstreamed). Will continue to work if/when pip2/3
emit the correct output. pip is already very slow at printing the
completions (see #4448) so the `sed` call overhead is neglible.

share/completions/pip2.fish

@@ -1,3 +1,3 @@
 if command -sq pip2
-    pip2 completion --fish | source
+    pip2 completion --fish | sed 's/-c pip/-c pip2/' | source
 end

share/completions/pip3.fish

@@ -1,3 +1,3 @@
 if command -sq pip3
-    pip3 completion --fish | source
+    pip3 completion --fish | sed 's/-c pip/-c pip3/' | source
 end