submodule: correctly delimit the keys to use for lookup

The regex we use to look at the gitmodules file does not correctly
delimit the name of submodule which we want to look up and puts '.*'
straight after the name, maching on any submodule which has the seeked
submodule as a prefix of its name.

Add the missing '\.' in the regex so we want a full stop to exist both
before and after the submodule name.

src/submodule.c

@@ -1385,7 +1385,7 @@ int git_submodule_reload(git_submodule *sm, int force)
 
 		git_buf_sets(&path, "submodule\\.");
 		git_buf_text_puts_escape_regex(&path, sm->name);
-		git_buf_puts(&path, ".*");
+		git_buf_puts(&path, "\\..*");
 
 		if (git_buf_oom(&path)) {
 			error = -1;