Don't permit ref/branch names to end with ".lock"

We already skip over loose refs under $GIT_DIR/refs if the name
ends with ".lock", so creating a branch named "foo.lock" will not
appear in the output of "git branch", "git for-each-ref", nor will
its commit be considered reachable by "git rev-list --all".

In the latter case this is especially evil, as it may cause
repository corruption when objects reachable only through such a
ref are deleted by "git prune".

It should be reasonably safe to deny use of ".lock" as a ref suffix.
In prior versions of Git such branches would be "phantom branches";
you can create it, but you can't see it in "git branch" output.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -34,6 +34,8 @@ imposes the following rules on how refs are named:
 
 . They cannot end with a slash `/` nor a dot `.`.
 
+. They cannot end with the sequence `.lock`.
+
 . They cannot contain a sequence `@{`.
 
 These rules makes it easy for shell script based tools to parse

refs.c

@@ -676,6 +676,7 @@ int for_each_rawref(each_ref_fn fn, void *cb_data)
  * - it has double dots "..", or
  * - it has ASCII control character, "~", "^", ":" or SP, anywhere, or
  * - it ends with a "/".
+ * - it ends with ".lock"
  */
 
 static inline int bad_ref_char(int ch)
@@ -737,6 +738,8 @@ int check_ref_format(const char *ref)
 				return CHECK_REF_FORMAT_ERROR;
 			if (level < 2)
 				return CHECK_REF_FORMAT_ONELEVEL;
+			if (has_extension(ref, ".lock"))
+				return CHECK_REF_FORMAT_ERROR;
 			return ret;
 		}
 	}