diff --color-words breaks spacing

I noticed some weird spacing when comparing files with git diff
--color-words.  The space before a colored word disappears sometimes.

$ git --version
git version 2.11.0.485.g4e59582ff

echo "FOO foo; foo = bar" > a
echo "FOO foo = baz" > b
git diff --color-words --no-index a b
FOOfoo; foo = barbaz

There should be a space after FOO in the diff, even if git doesn't
think "foo" and "foo;" are the same word.

If I remove the semicolon, it looks better, but in fact it only moves
the error later. The missing space is now between the two "foo" words.

echo "FOO foo foo = bar" > a
echo "FOO foo = baz" > b
git diff --color-words --no-index a b
FOO foofoo = barbaz

Here's the same with the color codes changed to text for purposes of this email:

echo "FOO foo; foo = bar" > a
echo "FOO foo = baz" > b
git diff --color-words --no-index a b | tr  \\033 E
FOOE[31mfoo;E[m foo = E[31mbarE[mE[32mbazE[m

echo "FOO foo foo = bar" > a
echo "FOO foo = baz" > b
git diff --color-words --no-index a b | tr  \\033 E
FOO fooE[31mfooE[m = E[31mbarE[mE[32mbazE[m