git-subtree: Avoid using echo -n even indirectly

Since say uses echo, this uses echo -n, which is not portable - see
19c3c5fdcb35b66b792534c5dc4e8d87a3952d2a.

Without this commit, the output looks like:

...
-n 1891/    1936 (1883)
-n 1892/    1936 (1884)
-n 1893/    1936 (1885)
...

Signed-off-by: Paolo G. Giarrusso <p.giarrusso@gmail.com>