help: add help_unknown_ref()

When the user gives an unknown string to a command that expects to
get a ref, we could be more helpful than just saying "that's not a
ref" and die.

Add helper function help_unknown_ref() to take care of displaying an
error message along with a list of suggested refs the user might
have meant.  An interaction with "git merge" might go like this:

	$ git merge foo
	merge: foo - not something we can merge

	Did you mean one of these?
	    origin/foo
	    upstream/foo

Signed-off-by: Vikrant Varma <vikrant.varma94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -7,6 +7,7 @@
 #include "string-list.h"
 #include "column.h"
 #include "version.h"
+#include "refs.h"
 
 void add_cmdname(struct cmdnames *cmds, const char *name, int len)
 {
@@ -404,3 +405,52 @@ int cmd_version(int argc, const char **argv, const char *prefix)
 	printf("git version %s\n", git_version_string);
 	return 0;
 }
+
+struct similar_ref_cb {
+	const char *base_ref;
+	struct string_list *similar_refs;
+};
+
+static int append_similar_ref(const char *refname, const unsigned char *sha1,
+			      int flags, void *cb_data)
+{
+	struct similar_ref_cb *cb = (struct similar_ref_cb *)(cb_data);
+	char *branch = strrchr(refname, '/') + 1;
+	/* A remote branch of the same name is deemed similar */
+	if (!prefixcmp(refname, "refs/remotes/") &&
+	    !strcmp(branch, cb->base_ref))
+		string_list_append(cb->similar_refs,
+				   refname + strlen("refs/remotes/"));
+	return 0;
+}
+
+static struct string_list guess_refs(const char *ref)
+{
+	struct similar_ref_cb ref_cb;
+	struct string_list similar_refs = STRING_LIST_INIT_NODUP;
+
+	ref_cb.base_ref = ref;
+	ref_cb.similar_refs = &similar_refs;
+	for_each_ref(append_similar_ref, &ref_cb);
+	return similar_refs;
+}
+
+void help_unknown_ref(const char *ref, const char *cmd, const char *error)
+{
+	int i;
+	struct string_list suggested_refs = guess_refs(ref);
+
+	fprintf_ln(stderr, _("%s: %s - %s"), cmd, ref, error);
+
+	if (suggested_refs.nr > 0) {
+		fprintf_ln(stderr,
+			   Q_("\nDid you mean this?",
+			      "\nDid you mean one of these?",
+			      suggested_refs.nr));
+		for (i = 0; i < suggested_refs.nr; i++)
+			fprintf(stderr, "\t%s\n", suggested_refs.items[i].string);
+	}
+
+	string_list_clear(&suggested_refs, 0);
+	exit(1);
+}

help.h

@@ -27,4 +27,9 @@ extern void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes);
 extern int is_in_cmdlist(struct cmdnames *cmds, const char *name);
 extern void list_commands(unsigned int colopts, struct cmdnames *main_cmds, struct cmdnames *other_cmds);
 
+/*
+ * call this to die(), when it is suspected that the user mistyped a
+ * ref to the command, to give suggested "correct" refs.
+ */
+extern void help_unknown_ref(const char *ref, const char *cmd, const char *error);
 #endif /* HELP_H */