Make the default abbrev length configurable

The default of 7 comes from fairly early in git development, when
seven hex digits was a lot (it covers about 250+ million hash
values). Back then I thought that 65k revisions was a lot (it was what
we were about to hit in BK), and each revision tends to be about 5-10
new objects or so, so a million objects was a big number.

These days, the kernel isn't even the largest git project, and even
the kernel has about 220k revisions (_much_ bigger than the BK tree
ever was) and we are approaching two million objects. At that point,
seven hex digits is still unique for a lot of them, but when we're
talking about just two orders of magnitude difference between number
of objects and the hash size, there _will_ be collisions in truncated
hash values. It's no longer even close to unrealistic - it happens all
the time.

We should both increase the default abbrev that was unrealistically
small, _and_ add a way for people to set their own default per-project
in the git config file.

This is the first step to first make it configurable; the default of 7
is not raised yet.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/cache.h b/cache.h
index d83d68c859..8d73d88962 100644
--- a/cache.h
+++ b/cache.h
@@ -540,6 +540,7 @@ extern int trust_executable_bit;
 extern int trust_ctime;
 extern int quote_path_fully;
 extern int has_symlinks;
+extern int minimum_abbrev, default_abbrev;
 extern int ignore_case;
 extern int assume_unchanged;
 extern int prefer_symlink_refs;
@@ -758,8 +759,8 @@ static inline unsigned int hexval(unsigned char c)
 }
 
 /* Convert to/from hex/sha1 representation */
-#define MINIMUM_ABBREV 4
-#define DEFAULT_ABBREV 7
+#define MINIMUM_ABBREV minimum_abbrev
+#define DEFAULT_ABBREV default_abbrev
 
 struct object_context {
 	unsigned char tree[20];