xbps-src: use HEAD commit time for SOURCE_DATE_EPOCH

In order to make builds more reproducible SOURCE_DATE_EPOCH was set to
the time of the last commit that touched the template. Since trying to
reproduce a build from a different revision is futile (the most obvious
reason is that the source-revisions property includes the HEAD commit
hash) and looking up the commit in question can take several seconds,
stop wasting time an just use HEAD.

Closes: #12314 [via git-merge-pr]

common/environment/setup/git.sh

@@ -14,6 +14,6 @@ if [ -z "${SOURCE_DATE_EPOCH}" -a -n "$IN_CHROOT" ]; then
 	if $GIT_CMD -C ${XBPS_SRCPKGDIR}/${basepkg} status -u normal --porcelain template | grep "^?? " &> /dev/null; then
 		export SOURCE_DATE_EPOCH="$(stat -c %Y ${XBPS_SRCPKGDIR}/${basepkg}/template)"
 	else
-		export SOURCE_DATE_EPOCH="$($GIT_CMD -C ${XBPS_SRCPKGDIR}/${basepkg} log --pretty='%ct' -n1 .)"
+		export SOURCE_DATE_EPOCH="$($GIT_CMD log --pretty='%ct' -n1 HEAD)"
 	fi
 fi

etc/defaults.conf

@@ -121,6 +121,6 @@ XBPS_SUCMD="sudo /bin/sh -c"
 
 # [OPTIONAL]
 # Enable to use the standard mtime of files. Otherwise it will be rewritten to
-# the last commit time of the package.
+# the HEAD commit time.
 #
 #XBPS_USE_BUILD_MTIME=yes