Move create_branch into a library file

You can also create branches, in exactly the same way, with checkout -b.

This introduces branch.{c,h} library files for doing porcelain-level
operations on branches (such as creating them with their appropriate
default configuration).

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>

branch.h

@@ -0,0 +1,8 @@
+#ifndef BRANCH_H
+#define BRANCH_H
+
+void create_branch(const char *head,
+		   const char *name, const char *start_name,
+		   int force, int reflog, int track);
+
+#endif