Set git commit dates based on $SOURCE_DATE_EPOCH or $RPM_BUILD_TIME

(https://github.com/rpm-software-management/rpm/pull/2933 is required
for $RPM_BUILD_TIME.)

When %autosetup -S git is used, a bunch of git commits are created. Without an
override, their metadata will include the current time. Many projects include
the git hash of HEAD in the build information, which means that the build result
is not reproducible. Since we're creating a scratch repo ourselves, this hash
doesn't have any meaning outside of the build. If $SOURCE_DATE_EPOCH is set, use
that as the commit timestamp instead. If $SOURCE_DATE_EPOCH is _not_ set, fall
back to $RPM_BUILD_TIME, which should always be set.

For projects which somehow make use of the git commit information, this fixes
an irreproducibility. For projects which don't care about the git hash, this
should make no difference.

Fixes https://pagure.io/fedora-reproducible-builds/project/issue/9.

macros.in

@@ -1218,6 +1218,8 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
 %__scm_apply_hg(qp:m:)\
 %{__hg} import - %{-p:-p%{-p*}} %{-q} -m %{-m*} --user "%{__scm_author}"
 
+%__scm_source_timestamp @${SOURCE_DATE_EPOCH:-${RPM_BUILD_TIME:?}}
+
 # Git
 %__scm_setup_git(q)\
 %{__git} init %{-q}\
@@ -1225,12 +1227,14 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
 %{__git} config user.email "%{__scm_usermail}"\
 %{__git} config gc.auto 0\
 %{__git} add --force .\
+GIT_COMMITTER_DATE=%{__scm_source_timestamp} GIT_AUTHOR_DATE=%{__scm_source_timestamp}\\\
 	%{__git} commit %{-q} --allow-empty -a\\\
 	--author "%{__scm_author}" -m "%{NAME}-%{VERSION} base"\
 %{__git} checkout --track -b rpm-build
 
 %__scm_apply_git(qp:m:)\
 %{__git} apply --index --reject %{-p:-p%{-p*}} -\
+GIT_COMMITTER_DATE=%{__scm_source_timestamp} GIT_AUTHOR_DATE=%{__scm_source_timestamp}\\\
 	%{__git} commit %{-q} -m %{-m*} --author "%{__scm_author}"
 
 # Git, using "git am" (-m is unused)
@@ -1238,6 +1242,7 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
 %{expand:%__scm_setup_git %{-q}}
 
 %__scm_apply_git_am(qp:m:)\
+GIT_COMMITTER_DATE=%{__scm_source_timestamp}\\\
 	%{__git} am --reject %{-q} %{-p:-p%{-p*}}
 
 # Quilt