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, 8 insertions, 0 deletions

diff --git a/config.c b/config.c
index 625e051876..e8a6e56795 100644
--- a/config.c
+++ b/config.c
@@ -531,6 +531,14 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.abbrevlength")) {
+		int abbrev = git_config_int(var, value);
+		if (abbrev < minimum_abbrev || abbrev > 40)
+			return -1;
+		default_abbrev = abbrev;
+		return 0;
+	}
+
 	if (!strcmp(var, "core.loosecompression")) {
 		int level = git_config_int(var, value);
 		if (level == -1)