config.c: create missing parent directories when modifying config files

'git config' (--add / --unset etc.) automatically creates missing config
files. However, it fails with a misleading error message "could not lock
config file" if the parent directory doesn't exist.

Also create missing parent directories.

This is particularly important when calling

	git config -f /non/existing/directory/config ...

We *must not* create the leading directories when locking a config file.
It is the caller's responsibility to ensure that the directory exists,
just like it is the caller's responsibility to call `git init` before
running repository operations.

Point in case: if we simply create all leading directories, calling
`git config user.name me` *outside* of a Git worktree will *create*
.git/!

This fixes git-for-windows#643 and
https://groups.google.com/forum/#!topic/git-for-windows/fVRdnDIKVuw

[jes: prevented bogus .git/ directories from being created]

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: kblees

config.c

@@ -1952,6 +1952,20 @@ int git_config_key_is_valid(const char *key)
 	return !git_config_parse_key_1(key, NULL, NULL, 1);
 }
 
+static int lock_config_file(const char *config_filename,
+		struct lock_file **result)
+{
+	int fd;
+
+	*result = xcalloc(1, sizeof(struct lock_file));
+	fd = hold_lock_file_for_update(*result, config_filename, 0);
+	if (fd < 0)
+		error("could not lock config file %s: %s", config_filename,
+				strerror(errno));
+
+	return fd;
+}
+
 /*
  * If value==NULL, unset in (remove from) config,
  * if value_regex!=NULL, disregard key/value pairs where value does not match.
@@ -2003,10 +2017,8 @@ int git_config_set_multivar_in_file_gently(const char *config_filename,
 	 * The lock serves a purpose in addition to locking: the new
 	 * contents of .git/config will be written into it.
 	 */
-	lock = xcalloc(1, sizeof(struct lock_file));
-	fd = hold_lock_file_for_update(lock, config_filename, 0);
+	fd = lock_config_file(config_filename, &lock);
 	if (fd < 0) {
-		error("could not lock config file %s: %s", config_filename, strerror(errno));
 		free(store.key);
 		ret = CONFIG_NO_LOCK;
 		goto out_free;
@@ -2305,12 +2317,9 @@ int git_config_rename_section_in_file(const char *config_filename,
 	if (!config_filename)
 		config_filename = filename_buf = git_pathdup("config");
 
-	lock = xcalloc(1, sizeof(struct lock_file));
-	out_fd = hold_lock_file_for_update(lock, config_filename, 0);
-	if (out_fd < 0) {
-		ret = error("could not lock config file %s", config_filename);
+	out_fd = lock_config_file(config_filename, &lock);
+	if (out_fd < 0)
 		goto out;
-	}
 
 	if (!(config_file = fopen(config_filename, "rb"))) {
 		/* no config file means nothing to rename, no error */