core.abbrevguard: Ensure short object names stay unique a bit longer

Even though git makes sure that it uses enough hexdigits to show an
abbreviated object name unambiguously, as more objects are added to the
repository over time, a short name that used to be unique will stop being
unique.  Git uses this many extra hexdigits that are more than necessary
to make the object name currently unique, in the hope that its output will
stay unique a bit longer.

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

Documentation/config.txt

@@ -374,6 +374,15 @@ core.warnAmbiguousRefs::
 	If true, git will warn you if the ref name you passed it is ambiguous
 	and might match multiple refs in the .git/refs/ tree. True by default.
 
+core.abbrevguard::
+	Even though git makes sure that it uses enough hexdigits to show
+	an abbreviated object name unambiguously, as more objects are
+	added to the repository over time, a short name that used to be
+	unique will stop being unique.  Git uses this many extra hexdigits
+	that are more than necessary to make the object name currently
+	unique, in the hope that its output will stay unique a bit longer.
+	Defaults to 0.
+
 core.compression::
 	An integer -1..9, indicating a default compression level.
 	-1 is the zlib default. 0 means no compression,

cache.h

@@ -545,6 +545,7 @@ extern int assume_unchanged;
 extern int prefer_symlink_refs;
 extern int log_all_ref_updates;
 extern int warn_ambiguous_refs;
+extern int unique_abbrev_extra_length;
 extern int shared_repository;
 extern const char *apply_default_whitespace;
 extern const char *apply_default_ignorewhitespace;

config.c

@@ -489,6 +489,13 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.abbrevguard")) {
+		unique_abbrev_extra_length = git_config_int(var, value);
+		if (unique_abbrev_extra_length < 0)
+			unique_abbrev_extra_length = 0;
+		return 0;
+	}
+
 	if (!strcmp(var, "core.bare")) {
 		is_bare_repository_cfg = git_config_bool(var, value);
 		return 0;

environment.c

@@ -21,6 +21,7 @@ int prefer_symlink_refs;
 int is_bare_repository_cfg = -1; /* unspecified */
 int log_all_ref_updates = -1; /* unspecified */
 int warn_ambiguous_refs = 1;
+int unique_abbrev_extra_length;
 int repository_format_version;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;

sha1_name.c

@@ -206,7 +206,9 @@ const char *find_unique_abbrev(const unsigned char *sha1, int len)
 		if (exists
 		    ? !status
 		    : status == SHORT_NAME_NOT_FOUND) {
-			hex[len] = 0;
+			int cut_at = len + unique_abbrev_extra_length;
+			cut_at = (cut_at < 40) ? cut_at : 40;
+			hex[cut_at] = 0;
 			return hex;
 		}
 		len++;