Merge branch 'jc/abbrev-guard'

* jc/abbrev-guard:
  core.abbrevguard: Ensure short object names stay unique a bit longer

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++;