update-server-info: avoid needless overwrites

Do not change the existing info/refs and objects/info/packs
files if they match the existing content on the filesystem.
This is intended to preserve mtime and make it easier for dumb
HTTP pollers to rely on the If-Modified-Since header.

Combined with stdio and kernel buffering; the kernel should be
able to avoid block layer writes and reduce wear for small files.

As a result, the --force option is no longer needed.  So stop
documenting it, but let it remain for compatibility (and
debugging, if necessary).

v3: perform incremental comparison while generating to avoid
    OOM with giant files.  Remove documentation for --force.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5200-update-server-info.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+
+test_description='Test git update-server-info'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' 'test_commit file'
+
+test_expect_success 'create info/refs' '
+	git update-server-info &&
+	test_path_is_file .git/info/refs
+'
+
+test_expect_success 'modify and store mtime' '
+	test-tool chmtime =0 .git/info/refs &&
+	test-tool chmtime --get .git/info/refs >a
+'
+
+test_expect_success 'info/refs is not needlessly overwritten' '
+	git update-server-info &&
+	test-tool chmtime --get .git/info/refs >b &&
+	test_cmp a b
+'
+
+test_expect_success 'info/refs can be forced to update' '
+	git update-server-info -f &&
+	test-tool chmtime --get .git/info/refs >b &&
+	! test_cmp a b
+'
+
+test_expect_success 'info/refs updates when changes are made' '
+	test-tool chmtime =0 .git/info/refs &&
+	test-tool chmtime --get .git/info/refs >b &&
+	test_cmp a b &&
+	git update-ref refs/heads/foo HEAD &&
+	git update-server-info &&
+	test-tool chmtime --get .git/info/refs >b &&
+	! test_cmp a b
+'
+
+test_done