builtin-fetch: add --dry-run option

Teach fetch --dry-run as users of "git remote prune" switching to "git fetch
--prune" may expect it. Unfortunately OPT__DRY_RUN() cannot be used as fetch
already uses "-n" for something else.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-fetch.c b/builtin-fetch.c
index 81974ea..76ec5ea 100644
--- a/builtin-fetch.c
+++ b/builtin-fetch.c
@@ -26,7 +26,7 @@
 	TAGS_SET = 2
 };
 
-static int all, append, force, keep, multiple, prune, update_head_ok, verbosity;
+static int all, append, dry_run, force, keep, multiple, prune, update_head_ok, verbosity;
 static int tags = TAGS_DEFAULT;
 static const char *depth;
 static const char *upload_pack;
@@ -51,6 +51,8 @@
 		    "do not fetch all tags (--no-tags)", TAGS_UNSET),
 	OPT_BOOLEAN('p', "prune", &prune,
 		    "prune tracking branches no longer on remote"),
+	OPT_BOOLEAN(0, "dry-run", &dry_run,
+		    "dry run"),
 	OPT_BOOLEAN('k', "keep", &keep, "keep downloaded pack"),
 	OPT_BOOLEAN('u', "update-head-ok", &update_head_ok,
 		    "allow updating of HEAD ref"),
@@ -187,6 +189,8 @@
 	char *rla = getenv("GIT_REFLOG_ACTION");
 	static struct ref_lock *lock;
 
+	if (dry_run)
+		return 0;
 	if (!rla)
 		rla = default_rla.buf;
 	snprintf(msg, sizeof(msg), "%s: %s", rla, action);
@@ -312,7 +316,7 @@
 	char note[1024];
 	const char *what, *kind;
 	struct ref *rm;
-	char *url, *filename = git_path("FETCH_HEAD");
+	char *url, *filename = dry_run ? "/dev/null" : git_path("FETCH_HEAD");
 
 	fp = fopen(filename, "a");
 	if (!fp)
@@ -617,7 +621,7 @@
 		die("Don't know how to fetch from %s", transport->url);
 
 	/* if not appending, truncate FETCH_HEAD */
-	if (!append) {
+	if (!append && !dry_run) {
 		char *filename = git_path("FETCH_HEAD");
 		FILE *fp = fopen(filename, "w");
 		if (!fp)
@@ -725,9 +729,11 @@
 static int fetch_multiple(struct string_list *list)
 {
 	int i, result = 0;
-	const char *argv[] = { "fetch", NULL, NULL, NULL, NULL, NULL };
+	const char *argv[] = { "fetch", NULL, NULL, NULL, NULL, NULL, NULL };
 	int argc = 1;
 
+	if (dry_run)
+		argv[argc++] = "--dry-run";
 	if (prune)
 		argv[argc++] = "--prune";
 	if (verbosity >= 2)