[PATCH] lei_mirror: use --no-write-fetch-head on git 2.29+

[Eric Wong <e@80x24.org>]

This avoids unnecessary writes to the FETCH_HEAD file, which is
worthless in multi-remote mirrors.  Actually, I haven't found
FETCH_HEAD useful anywhere since the `/remotes/' namespace
became popular...
---
 lib/PublicInbox/Git.pm       | 5 +++++
 lib/PublicInbox/LeiMirror.pm | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index fd7a0382..2729b38f 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -52,6 +52,11 @@ my %ESC_GIT = map { $GIT_ESC{$_} => $_ } keys %GIT_ESC;
 my $EXE_ST = ''; # pack('dd', st_ctime, st_size);
 my ($GIT_EXE, $GIT_VER);
 
+sub version {
+	check_git_exe();
+	$GIT_VER;
+}
+
 sub check_git_exe () {
 	$GIT_EXE = which('git') // die "git not found in $ENV{PATH}";
 	my @st = stat($GIT_EXE) or die "stat: $!";
diff --git a/lib/PublicInbox/LeiMirror.pm b/lib/PublicInbox/LeiMirror.pm
index ec2b56df..4c564c84 100644
--- a/lib/PublicInbox/LeiMirror.pm
+++ b/lib/PublicInbox/LeiMirror.pm
@@ -16,6 +16,7 @@ use Carp qw(croak);
 use URI;
 use PublicInbox::Config;
 use PublicInbox::Inbox;
+use PublicInbox::Git;
 use PublicInbox::LeiCurl;
 use PublicInbox::OnDestroy;
 use PublicInbox::SHA qw(sha256_hex sha1_hex);
@@ -275,6 +276,8 @@ sub fetch_args ($$) {
 			($lei->{opt}->{jobs} // 1) > 1;
 	push @cmd, '-v' if $lei->{opt}->{verbose};
 	push(@cmd, '-p') if $lei->{opt}->{prune};
+	PublicInbox::Git::version() >= ((2 << 24) | (29 << 16)) and
+		push(@cmd, '--no-write-fetch-head');
 	@cmd;
 }