Update contributors.sh script not to count publish commits

Whenever you run `./pants publish.jar ...` it can add multiple commits to the commit log of the form:

```
pants build committing publish data for push of org.pantsbuild#...
```

These aren't very useful for counting real contributions at the end of a release cycle, so let's exclude them from the count.

Testing Done:
I didn't run CI because this script doesn't get executed as a part of CI

Before:

```
$ build-support/bin/contributors.sh -s HEAD~30
  13 Eric Ayers
   4 Kris Wilson
   3 Stu Hood
   3 Chris Pesto
   2 Yi Cheng
   2 Garrett Malmquist
   1 Wolfram Arnold
   1 Bruno Bieth
   1 Benjy Weinberger
```

After:

```
build-support/bin/contributors.sh -s HEAD~30
   6 Eric Ayers
   4 Kris Wilson
   3 Stu Hood
   3 Chris Pesto
   2 Garrett Malmquist
   1 Yi Cheng
   1 Wolfram Arnold
   1 Bruno Bieth
   1 Benjy Weinberger
```

Reviewed at https://rbcommons.com/s/twitter/r/3946/

build-support/bin/contributors.sh

@@ -38,8 +38,13 @@ done
 function contributors() {
   range="${1:-HEAD}"
 
-  # Include all commits in range but exclude all commits from the imported zinc tree.
-  git log --use-mailmap --format=format:%aN ${range} ^imported_zinc_tree
+  # Include all commits in range but exclude:
+  #  - all commits from the imported zinc tree.
+  #  - commits that are from running the publish goal
+  git log --use-mailmap --format="format:%aN;%s" ${range} ^imported_zinc_tree \
+    | grep -v "pants build committing publish data" \
+    | sed -e 's/;.*$//'
+
 }
 
 if [[ -n "${since}" ]]; then