read_gitfile_gently(): rename misnamed function to read_gitfile()

The function was not gentle at all to the callers and died without giving
them a chance to deal with possible errors. Rename it to read_gitfile(),
and update all the callers.

As no existing caller needs a true "gently" variant, we do not bother
adding one at this point.

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

refs.c

@@ -451,7 +451,7 @@ int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *re
 	memcpy(gitdir + len, "/.git", 6);
 	len += 5;
 
-	tmp = read_gitfile_gently(gitdir);
+	tmp = read_gitfile(gitdir);
 	if (tmp) {
 		free(gitdir);
 		len = strlen(tmp);