refspec: add support for negative refspecs

Both fetch and push support pattern refspecs which allow fetching or
pushing references that match a specific pattern. Because these patterns
are globs, they have somewhat limited ability to express more complex
situations.

For example, suppose you wish to fetch all branches from a remote except
for a specific one. To allow this, you must setup a set of refspecs
which match only the branches you want. Because refspecs are either
explicit name matches, or simple globs, many patterns cannot be
expressed.

Add support for a new type of refspec, referred to as "negative"
refspecs. These are prefixed with a '^' and mean "exclude any ref
matching this refspec". They can only have one "side" which always
refers to the source. During a fetch, this refers to the name of the ref
on the remote. During a push, this refers to the name of the ref on the
local side.

With negative refspecs, users can express more complex patterns. For
example:

 git fetch origin refs/heads/*:refs/remotes/origin/* ^refs/heads/dontwant

will fetch all branches on origin into remotes/origin, but will exclude
fetching the branch named dontwant.

Refspecs today are commutative, meaning that order doesn't expressly
matter. Rather than forcing an implied order, negative refspecs will
always be applied last. That is, in order to match, a ref must match at
least one positive refspec, and match none of the negative refspecs.
This is similar to how negative pathspecs work.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 6 deletions

diff --git a/refspec.h b/refspec.h
index 8d654e3..e5bf6d2 100644
--- a/refspec.h
+++ b/refspec.h
@@ -5,12 +5,13 @@
 extern const struct refspec_item *tag_refspec;
 
 /**
- * A struct refspec_item holds the parsed interpretation of a refspec.  If it will
- * force updates (starts with a '+'), force is true.  If it is a pattern
- * (sides end with '*') pattern is true.  src and dest are the two sides
- * (including '*' characters if present); if there is only one side, it is src,
- * and dst is NULL; if sides exist but are empty (i.e., the refspec either
- * starts or ends with ':'), the corresponding side is "".
+ * A struct refspec_item holds the parsed interpretation of a refspec.  If it
+ * will force updates (starts with a '+'), force is true.  If it is a pattern
+ * (sides end with '*') pattern is true.  If it is a negative refspec, (starts
+ * with '^'), negative is true.  src and dest are the two sides (including '*'
+ * characters if present); if there is only one side, it is src, and dst is
+ * NULL; if sides exist but are empty (i.e., the refspec either starts or ends
+ * with ':'), the corresponding side is "".
  *
  * remote_find_tracking(), given a remote and a struct refspec_item with either src
  * or dst filled out, will fill out the other such that the result is in the
@@ -22,6 +23,7 @@ struct refspec_item {
 	unsigned pattern : 1;
 	unsigned matching : 1;
 	unsigned exact_sha1 : 1;
+	unsigned negative : 1;
 
 	char *src;
 	char *dst;