sha1_file: stop opening files with O_NOATIME

When we open object files, we try to do so with O_NOATIME.
This dates back to 144bde78 (Use O_NOATIME when opening
the sha1 files., 2005-04-23), which is an optimization to
avoid creating a bunch of dirty inodes when we're accessing
many objects.  But a few things have changed since then:

  1. In June 2005, git learned about packfiles, which means
     we would do a lot fewer atime updates (rather than one
     per object access, we'd generally get one per packfile).

  2. In late 2006, Linux learned about "relatime", which is
     generally the default on modern installs. So
     performance around atimes updates is a non-issue there
     these days.

     All the world isn't Linux, but as it turns out, Linux
     is the only platform to implement O_NOATIME in the
     first place.

So it's very unlikely that this code is helping anybody
these days.
Helped-by: NJeff King <peff@peff.net>
[jc: took idea and log message from peff]
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -1123,7 +1123,7 @@ extern int hash_sha1_file_literally(const void *buf, unsigned long len, const ch
 extern int pretend_sha1_file(void *, unsigned long, enum object_type, unsigned char *);
 extern int force_object_loose(const unsigned char *sha1, time_t mtime);
 extern int git_open_cloexec(const char *name, int flags);
-extern int git_open(const char *name);
+#define git_open(name) git_open_cloexec(name, O_RDONLY)
 extern void *map_sha1_file(const unsigned char *sha1, unsigned long *size);
 extern int unpack_sha1_header(git_zstream *stream, unsigned char *map, unsigned long mapsize, void *buffer, unsigned long bufsiz);
 extern int parse_sha1_header(const char *hdr, unsigned long *sizep);

sha1_file.c

@@ -27,14 +27,6 @@
 #include "list.h"
 #include "mergesort.h"
 
-#ifndef O_NOATIME
-#if defined(__linux__) && (defined(__i386__) || defined(__PPC__))
-#define O_NOATIME 01000000
-#else
-#define O_NOATIME 0
-#endif
-#endif
-
 #define SZ_FMT PRIuMAX
 static inline uintmax_t sz_fmt(size_t s) { return s; }
 
@@ -1586,19 +1578,6 @@ int git_open_cloexec(const char *name, int flags)
 	return fd;
 }
 
-int git_open(const char *name)
-{
-	static int noatime = O_NOATIME;
-	int fd = git_open_cloexec(name, O_RDONLY);
-
-	if (0 <= fd && (noatime & O_NOATIME)) {
-		int flags = fcntl(fd, F_GETFL);
-		if (fcntl(fd, F_SETFL, flags | noatime))
-			noatime = 0;
-	}
-	return fd;
-}
-
 static int stat_sha1_file(const unsigned char *sha1, struct stat *st)
 {
 	struct alternate_object_database *alt;