Create internal strcmp variants for function ptrs

Using the builtin strcmp and strcasecmp as function pointers is
problematic on win32.  This adds internal implementations and
divorces us from the platform linkage.

src/diff.c

@@ -551,15 +551,15 @@ static int diff_list_init_from_iterators(
 	if (!old_iter->ignore_case && !new_iter->ignore_case) {
 		diff->opts.flags &= ~GIT_DIFF_DELTAS_ARE_ICASE;
 
-		diff->strcomp    = strcmp;
-		diff->strncomp   = strncmp;
+		diff->strcomp    = git__strcmp;
+		diff->strncomp   = git__strncmp;
 		diff->pfxcomp    = git__prefixcmp;
 		diff->entrycomp  = git_index_entry__cmp;
 	} else {
 		diff->opts.flags |= GIT_DIFF_DELTAS_ARE_ICASE;
 
-		diff->strcomp    = strcasecmp;
-		diff->strncomp   = strncasecmp;
+		diff->strcomp    = git__strcasecmp;
+		diff->strncomp   = git__strncasecmp;
 		diff->pfxcomp    = git__prefixcmp_icase;
 		diff->entrycomp  = git_index_entry__cmp_icase;
 	}

src/pathspec.c

@@ -122,11 +122,11 @@ bool git_pathspec_match_path(
 		fnmatch_flags = FNM_CASEFOLD;
 
 	if (casefold) {
-		use_strcmp  = strcasecmp;
-		use_strncmp = strncasecmp;
+		use_strcmp  = git__strcasecmp;
+		use_strncmp = git__strncasecmp;
 	} else {
-		use_strcmp  = strcmp;
-		use_strncmp = strncmp;
+		use_strcmp  = git__strcmp;
+		use_strncmp = git__strncmp;
 	}
 
 	git_vector_foreach(vspec, i, match) {

src/util.c

@@ -174,6 +174,36 @@ int git__strtol32(int32_t *result, const char *nptr, const char **endptr, int ba
 	return error;
 }
 
+int git__strcmp(const char *a, const char *b)
+{
+	while (*a && *b && *a == *b)
+		++a, ++b;
+	return (int)(*(const unsigned char *)a) - (int)(*(const unsigned char *)b);
+}
+
+int git__strcasecmp(const char *a, const char *b)
+{
+	while (*a && *b && tolower(*a) == tolower(*b))
+		++a, ++b;
+	return (tolower(*a) - tolower(*b));
+}
+
+int git__strncmp(const char *a, const char *b, size_t sz)
+{
+	while (sz && *a && *b && *a == *b)
+		--sz, ++a, ++b;
+	if (!sz)
+		return 0;
+	return (int)(*(const unsigned char *)a) - (int)(*(const unsigned char *)b);
+}
+
+int git__strncasecmp(const char *a, const char *b, size_t sz)
+{
+	while (sz && *a && *b && tolower(*a) == tolower(*b))
+		--sz, ++a, ++b;
+	return !sz ? 0 : (tolower(*a) - tolower(*b));
+}
+
 void git__strntolower(char *str, size_t len)
 {
 	size_t i;

src/util.h

@@ -134,6 +134,11 @@ extern int git__bsearch(
 
 extern int git__strcmp_cb(const void *a, const void *b);
 
+extern int git__strcmp(const char *a, const char *b);
+extern int git__strcasecmp(const char *a, const char *b);
+extern int git__strncmp(const char *a, const char *b, size_t sz);
+extern int git__strncasecmp(const char *a, const char *b, size_t sz);
+
 typedef struct {
 	short refcount;
 	void *owner;