checkout: factor out functions to new lib file

Factor the functions out, so they can be re-used from other places.  In
particular these functions will be re-used in builtin/worktree.c to make
git worktree add dwim more.

While there add some docs to the function.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -759,6 +759,7 @@ LIB_OBJS += branch.o
 LIB_OBJS += bulk-checkin.o
 LIB_OBJS += bundle.o
 LIB_OBJS += cache-tree.o
+LIB_OBJS += checkout.o
 LIB_OBJS += color.o
 LIB_OBJS += column.o
 LIB_OBJS += combine-diff.o

builtin/checkout.c

@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "config.h"
+#include "checkout.h"
 #include "lockfile.h"
 #include "parse-options.h"
 #include "refs.h"
@@ -872,46 +873,6 @@ static int git_checkout_config(const char *var, const char *value, void *cb)
 	return git_xmerge_config(var, value, NULL);
 }
 
-struct tracking_name_data {
-	/* const */ char *src_ref;
-	char *dst_ref;
-	struct object_id *dst_oid;
-	int unique;
-};
-
-static int check_tracking_name(struct remote *remote, void *cb_data)
-{
-	struct tracking_name_data *cb = cb_data;
-	struct refspec query;
-	memset(&query, 0, sizeof(struct refspec));
-	query.src = cb->src_ref;
-	if (remote_find_tracking(remote, &query) ||
-	    get_oid(query.dst, cb->dst_oid)) {
-		free(query.dst);
-		return 0;
-	}
-	if (cb->dst_ref) {
-		free(query.dst);
-		cb->unique = 0;
-		return 0;
-	}
-	cb->dst_ref = query.dst;
-	return 0;
-}
-
-static const char *unique_tracking_name(const char *name, struct object_id *oid)
-{
-	struct tracking_name_data cb_data = { NULL, NULL, NULL, 1 };
-	cb_data.src_ref = xstrfmt("refs/heads/%s", name);
-	cb_data.dst_oid = oid;
-	for_each_remote(check_tracking_name, &cb_data);
-	free(cb_data.src_ref);
-	if (cb_data.unique)
-		return cb_data.dst_ref;
-	free(cb_data.dst_ref);
-	return NULL;
-}
-
 static int parse_branchname_arg(int argc, const char **argv,
 				int dwim_new_local_branch_ok,
 				struct branch_info *new,

checkout.c

@@ -0,0 +1,43 @@
+#include "cache.h"
+#include "remote.h"
+#include "checkout.h"
+
+struct tracking_name_data {
+	/* const */ char *src_ref;
+	char *dst_ref;
+	struct object_id *dst_oid;
+	int unique;
+};
+
+static int check_tracking_name(struct remote *remote, void *cb_data)
+{
+	struct tracking_name_data *cb = cb_data;
+	struct refspec query;
+	memset(&query, 0, sizeof(struct refspec));
+	query.src = cb->src_ref;
+	if (remote_find_tracking(remote, &query) ||
+	    get_oid(query.dst, cb->dst_oid)) {
+		free(query.dst);
+		return 0;
+	}
+	if (cb->dst_ref) {
+		free(query.dst);
+		cb->unique = 0;
+		return 0;
+	}
+	cb->dst_ref = query.dst;
+	return 0;
+}
+
+const char *unique_tracking_name(const char *name, struct object_id *oid)
+{
+	struct tracking_name_data cb_data = { NULL, NULL, NULL, 1 };
+	cb_data.src_ref = xstrfmt("refs/heads/%s", name);
+	cb_data.dst_oid = oid;
+	for_each_remote(check_tracking_name, &cb_data);
+	free(cb_data.src_ref);
+	if (cb_data.unique)
+		return cb_data.dst_ref;
+	free(cb_data.dst_ref);
+	return NULL;
+}

checkout.h

@@ -0,0 +1,13 @@
+#ifndef CHECKOUT_H
+#define CHECKOUT_H
+
+#include "cache.h"
+
+/*
+ * Check if the branch name uniquely matches a branch name on a remote
+ * tracking branch.  Return the name of the remote if such a branch
+ * exists, NULL otherwise.
+ */
+extern const char *unique_tracking_name(const char *name, struct object_id *oid);
+
+#endif /* CHECKOUT_H */