core.abbrev=no disables abbreviations

This allows users to write hash-agnostic scripts and configs by
disabling abbreviations.  Using "-c core.abbrev=40" will be
insufficient with SHA-256, and "-c core.abbrev=64" won't work with
SHA-1 repos today.

Signed-off-by: Eric Wong <e@80x24.org>
[jc: tweaked implementation, added doc and a test]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/core.txt

@@ -625,4 +625,6 @@ core.abbrev::
 	computed based on the approximate number of packed objects
 	in your repository, which hopefully is enough for
 	abbreviated object names to stay unique for some time.
+	If set to "no", no abbreviation is made and the object names
+	are shown in their full length.
 	The minimum length is 4.

config.c

@@ -1217,6 +1217,8 @@ static int git_default_core_config(const char *var, const char *value, void *cb)
 			return config_error_nonbool(var);
 		if (!strcasecmp(value, "auto"))
 			default_abbrev = -1;
+		else if (!git_parse_maybe_bool_text(value))
+			default_abbrev = the_hash_algo->hexsz;
 		else {
 			int abbrev = git_config_int(var, value);
 			if (abbrev < minimum_abbrev || abbrev > the_hash_algo->hexsz)

t/t3200-branch.sh

@@ -305,7 +305,9 @@ test_expect_success 'git branch --list -v with --abbrev' '
 
 	git branch -v --list --no-abbrev t >actual.noabbrev &&
 	git branch -v --list --abbrev=0 t >actual.0abbrev &&
+	git -c core.abbrev=no branch -v --list t >actual.noabbrev-conf &&
 	test_cmp actual.noabbrev actual.0abbrev &&
+	test_cmp actual.noabbrev actual.noabbrev-conf &&
 
 	git branch -v --list --abbrev=36 t >actual.36abbrev &&
 	# how many hexdigits are used?