git-branch: add options and tests for branch renaming

Extend git-branch with the following options:

  git-branch -m|-M [<oldbranch>] newbranch

The -M variation is required to force renaming over an exsisting
branchname.

This also indroduces $GIT_DIR/RENAME_REF which is a "metabranch"
used when renaming branches. It will always hold the original sha1
for the latest renamed branch.

Additionally, if $GIT_DIR/logs/RENAME_REF exists, all branch rename
events are logged there.

Finally, some testcases are added to verify the new options.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.h

@@ -47,4 +47,7 @@ extern int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned
 /** Returns 0 if target has the right format for a ref. **/
 extern int check_ref_format(const char *target);
 
+/** rename ref, return 0 on success **/
+extern int rename_ref(const char *oldref, const char *newref);
+
 #endif /* REFS_H */