config: add repo_config_set_worktree_gently()

Some config settings, such as those for sparse-checkout, are likely
intended to only apply to one worktree at a time. To make this write
easier, add a new config API method, repo_config_set_worktree_gently().

This method will attempt to write to the worktree-specific config, but
will instead write to the common config file if worktree config is not
enabled.  The next change will introduce a consumer of this method.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -21,6 +21,7 @@
 #include "dir.h"
 #include "color.h"
 #include "refs.h"
+#include "worktree.h"
 
 struct config_source {
 	struct config_source *prev;
@@ -2884,6 +2885,20 @@ int git_config_set_gently(const char *key, const char *value)
 	return git_config_set_multivar_gently(key, value, NULL, 0);
 }
 
+int repo_config_set_worktree_gently(struct repository *r,
+				    const char *key, const char *value)
+{
+	/* Only use worktree-specific config if it is is already enabled. */
+	if (repository_format_worktree_config) {
+		char *file = repo_git_path(r, "config.worktree");
+		int ret = git_config_set_multivar_in_file_gently(
+					file, key, value, NULL, 0);
+		free(file);
+		return ret;
+	}
+	return repo_config_set_multivar_gently(r, key, value, NULL, 0);
+}
+
 void git_config_set(const char *key, const char *value)
 {
 	git_config_set_multivar(key, value, NULL, 0);
@@ -3181,14 +3196,28 @@ void git_config_set_multivar_in_file(const char *config_filename,
 int git_config_set_multivar_gently(const char *key, const char *value,
 				   const char *value_pattern, unsigned flags)
 {
-	return git_config_set_multivar_in_file_gently(NULL, key, value, value_pattern,
+	return repo_config_set_multivar_gently(the_repository, key, value,
-						      flags);
+					       value_pattern, flags);
+}
+
+int repo_config_set_multivar_gently(struct repository *r, const char *key,
+				    const char *value,
+				    const char *value_pattern, unsigned flags)
+{
+	char *file = repo_git_path(r, "config");
+	int res = git_config_set_multivar_in_file_gently(file,
+							 key, value,
+							 value_pattern,
+							 flags);
+	free(file);
+	return res;
 }
 
 void git_config_set_multivar(const char *key, const char *value,
 			     const char *value_pattern, unsigned flags)
 {
-	git_config_set_multivar_in_file(NULL, key, value, value_pattern,
+	git_config_set_multivar_in_file(git_path("config"),
+					key, value, value_pattern,
 					flags);
 }
 

config.h

@@ -253,6 +253,13 @@ void git_config_set_in_file(const char *, const char *, const char *);
 
 int git_config_set_gently(const char *, const char *);
 
+/**
+ * Write a config value that should apply to the current worktree. If
+ * extensions.worktreeConfig is enabled, then the write will happen in the
+ * current worktree's config. Otherwise, write to the common config file.
+ */
+int repo_config_set_worktree_gently(struct repository *, const char *, const char *);
+
 /**
  * write config values to `.git/config`, takes a key/value pair as parameter.
  */
@@ -281,6 +288,7 @@ int git_config_parse_key(const char *, char **, size_t *);
 
 int git_config_set_multivar_gently(const char *, const char *, const char *, unsigned);
 void git_config_set_multivar(const char *, const char *, const char *, unsigned);
+int repo_config_set_multivar_gently(struct repository *, const char *, const char *, const char *, unsigned);
 int git_config_set_multivar_in_file_gently(const char *, const char *, const char *, const char *, unsigned);
 
 /**