API: Check return value from $gitInfo->getHeadCommitDate()

action=query&meta=siteinfo&siprop=extensions calls
$gitInfo->getHeadCommitDate() to attempt to find the commit date, but
does not check whether that function returned false before using the
value. This resulted in displaying the current date for "vcs-date" when
the function failed.

Bug: 64821
Change-Id: Ic39a74abe5160b3b7fbfb7c15323328d6b317560
(cherry picked from commit 986d3ef2c13aeaf8d172e89c30c2c2f93d9f1b55)