Add missing reference to GIT_COMMITTER_DATE in git-commit-tree documentation

Signed-off-by: NJosh Triplett <josh@freedesktop.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-commit-tree.txt

@@ -60,6 +60,7 @@ either `.git/config` file, or using the following environment variables.
 	GIT_AUTHOR_DATE
 	GIT_COMMITTER_NAME
 	GIT_COMMITTER_EMAIL
+	GIT_COMMITTER_DATE
 
 (nb "<", ">" and "\n"s are stripped)
 

Documentation/git.txt

@@ -340,6 +340,7 @@ git Commits
 'GIT_AUTHOR_DATE'::
 'GIT_COMMITTER_NAME'::
 'GIT_COMMITTER_EMAIL'::
+'GIT_COMMITTER_DATE'::
 	see gitlink:git-commit-tree[1]
 
 git Diffs