.gitignore: ignore 'git-subtree' as a build artifact

Running `make -C contrib/subtree/ test` creates a `git-subtree` executable
in the root of the repo.  Add it to the .gitignore so that anyone hacking
on subtree won't have to deal with that noise.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -162,6 +162,7 @@
 /git-stripspace
 /git-submodule
 /git-submodule--helper
+/git-subtree
 /git-svn
 /git-switch
 /git-symbolic-ref