This reverts commit 6e9e0327b7d7f384d8a223b4bc40330ef3e7fb61. People
can prepare a text file with Subject: and From: headers and feed it to
"am" (pretending the file is a piece of e-mail), and have actually been
doing so. Strict checking for Date: breaks this established workflow,
which wants to record the time of the commit as the author time.