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>

Documentation/fetch-options.txt

@@ -12,6 +12,11 @@
 	`git clone` with `--depth=<depth>` option (see linkgit:git-clone[1])
 	by the specified number of commits.
 
+ifndef::git-pull[]
+--dry-run::
+	Show what would be done, without making any changes.
+endif::git-pull[]
+
 -f::
 --force::
 	When 'git-fetch' is used with `<rbranch>:<lbranch>`

builtin-fetch.c

@@ -26,7 +26,7 @@ enum {
 	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 @@ static struct option builtin_fetch_options[] = {
 		    "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 @@ static int s_update_ref(const char *action,
 	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 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 	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 @@ static int do_fetch(struct transport *transport,
 		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 add_remote_or_group(const char *name, struct string_list *list)
 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)