revert: Separate cmdline parsing from functional code

Currently, revert_or_cherry_pick sets up a default git config, parses
command-line arguments, before preparing to pick commits.  This makes
for a bad API as the central worry of callers is to assert whether or
not a conflict occured while cherry picking.  The current API is like:

  if (revert_or_cherry_pick(argc, argv, opts) < 0)
     print "Something failed, we're not sure what"

Simplify and rename revert_or_cherry_pick to pick_commits so that it
only has the responsibility of setting up the revision walker and
picking commits in a loop.  Transfer the remaining work to its
callers.  Now, the API is simplified as:

  if (parse_args(argc, argv, opts) < 0)
     print "Can't parse arguments"
  if (pick_commits(opts) < 0)
     print "Error encountered in picking machinery"

Later in the series, pick_commits will also serve as the starting
point for continuing a cherry-pick or revert.

Inspired-by: Christian Couder <chriscool@tuxfamily.org>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -563,16 +563,12 @@ static void read_and_refresh_cache(struct replay_opts *opts)
 	rollback_lock_file(&index_lock);
 }
 
-static int revert_or_cherry_pick(int argc, const char **argv,
-				struct replay_opts *opts)
+static int pick_commits(struct replay_opts *opts)
 {
 	struct rev_info revs;
 	struct commit *commit;
 
-	git_config(git_default_config, NULL);
 	setenv(GIT_REFLOG_ACTION, action_name(opts), 0);
-	parse_args(argc, argv, opts);
-
 	if (opts->allow_ff) {
 		if (opts->signoff)
 			die(_("cherry-pick --ff cannot be used with --signoff"));
@@ -605,7 +601,9 @@ int cmd_revert(int argc, const char **argv, const char *prefix)
 	if (isatty(0))
 		opts.edit = 1;
 	opts.action = REVERT;
-	return revert_or_cherry_pick(argc, argv, &opts);
+	git_config(git_default_config, NULL);
+	parse_args(argc, argv, &opts);
+	return pick_commits(&opts);
 }
 
 int cmd_cherry_pick(int argc, const char **argv, const char *prefix)
@@ -614,5 +612,7 @@ int cmd_cherry_pick(int argc, const char **argv, const char *prefix)
 
 	memset(&opts, 0, sizeof(opts));
 	opts.action = CHERRY_PICK;
-	return revert_or_cherry_pick(argc, argv, &opts);
+	git_config(git_default_config, NULL);
+	parse_args(argc, argv, &opts);
+	return pick_commits(&opts);
 }