packed_ref_cache: don't use mmap() for small files

Take a hint from commit ea68b0c (hash-object: don't use mmap() for
small files, 2010-02-21) and use read() instead of mmap() for small
packed-refs files.

This also fixes #1410
(where xmmap() returns NULL for zero length[1], for which munmap() later
fails).

Alternatively, we could simply check for NULL before munmap(), or
introduce xmunmap() that could be used together with xmmap(). However,
always setting snapshot->buf to a valid pointer, by relying on
xmalloc(0)'s fallback to 1-byte allocation, makes using snapshots
easier.

[1] Logic introduced in commit 9130ac1 (Better error messages for
    corrupt databases, 2007-01-11)

This was cherry-picked from upstream's `pu` branch so that the fix is
included in Git for Windows v2.16.0.

Signed-off-by: Kim Gybels <kgybels@infogroep.be>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: kgybels

refs/packed-backend.c

@@ -455,6 +455,8 @@ static void verify_buffer_safe(struct snapshot *snapshot)
 				 last_line, eof - last_line);
 }
 
+#define SMALL_FILE_SIZE (32*1024)
+
 /*
  * Depending on `mmap_strategy`, either mmap or read the contents of
  * the `packed-refs` file into the snapshot. Return 1 if the file
@@ -489,21 +491,17 @@ static int load_contents(struct snapshot *snapshot)
 		die_errno("couldn't stat %s", snapshot->refs->path);
 	size = xsize_t(st.st_size);
 
-	switch (mmap_strategy) {
-	case MMAP_NONE:
+	if (size <= SMALL_FILE_SIZE || mmap_strategy == MMAP_NONE) {
 		snapshot->buf = xmalloc(size);
 		bytes_read = read_in_full(fd, snapshot->buf, size);
 		if (bytes_read < 0 || bytes_read != size)
 			die_errno("couldn't read %s", snapshot->refs->path);
 		snapshot->eof = snapshot->buf + size;
 		snapshot->mmapped = 0;
-		break;
-	case MMAP_TEMPORARY:
-	case MMAP_OK:
+	} else {
 		snapshot->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
 		snapshot->eof = snapshot->buf + size;
 		snapshot->mmapped = 1;
-		break;
 	}
 	close(fd);