Make angle's commit.h depend on git head

This is for reproducible build of angle related libraries.

Bug: 828669
Change-Id: I807c8fb01b976f6e7a71477ee934b9e0b0cdc684
Reviewed-on: https://chromium-review.googlesource.com/1164814
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

BUILD.gn

@@ -379,6 +379,9 @@ if (angle_use_commit_id) {
       commit_id_output_file,
     ]
 
+    # commit id should depend on angle's HEAD revision
+    inputs = [ ".git/HEAD" ]
+
     args = [
       "gen",
       rebase_path(".", root_build_dir),