util: introduce `git__strlcmp`

Introduce a utility function that compares a NUL terminated string to a
possibly not-NUL terminated string with length.  This is similar to
`strncmp` but with an added check to ensure that the lengths match (not
just the `size` portion of the two strings).

src/util.h

@@ -168,6 +168,17 @@ extern int git__strncasecmp(const char *a, const char *b, size_t sz);
 
 extern int git__strcasesort_cmp(const char *a, const char *b);
 
+/*
+ * Compare some NUL-terminated `a` to a possibly non-NUL terminated
+ * `b` of length `b_len`; like `strncmp` but ensuring that
+ * `strlen(a) == b_len` as well.
+ */
+GIT_INLINE(int) git__strlcmp(const char *a, const char *b, size_t b_len)
+{
+	int cmp = strncmp(a, b, b_len);
+	return cmp ? cmp : (int)a[b_len];
+}
+
 typedef struct {
 	git_atomic32 refcount;
 	void *owner;

tests/core/string.c

@@ -123,3 +123,14 @@ void test_core_string__strcasecmp(void)
 	cl_assert(git__strcasecmp("et", "e\342\202\254ghi=") < 0);
 	cl_assert(git__strcasecmp("\303\215", "\303\255") < 0);
 }
+
+void test_core_string__strlcmp(void)
+{
+	const char foo[3] = { 'f', 'o', 'o' };
+
+	cl_assert(git__strlcmp("foo", "foo", 3) == 0);
+	cl_assert(git__strlcmp("foo", foo, 3) == 0);
+	cl_assert(git__strlcmp("foo", "foobar", 3) == 0);
+	cl_assert(git__strlcmp("foobar", "foo", 3) > 0);
+	cl_assert(git__strlcmp("foo", "foobar", 6) < 0);
+}