strbuf_realpath(): use platform-dependent API if available

Some platforms (e.g. Windows) provide API functions to resolve paths
much quicker. Let's offer a way to short-cut `strbuf_realpath()` on
those platforms.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

abspath.c

@@ -91,6 +91,9 @@ static char *strbuf_realpath_1(struct strbuf *resolved, const char *path,
 			goto error_out;
 	}
 
+	if (platform_strbuf_realpath(resolved, path))
+		return resolved->buf;
+
 	strbuf_addstr(&remaining, path);
 	get_root_part(resolved, &remaining);
 

git-compat-util.h

@@ -544,6 +544,10 @@ static inline int git_has_dir_sep(const char *path)
 #define query_user_email() NULL
 #endif
 
+#ifndef platform_strbuf_realpath
+#define platform_strbuf_realpath(resolved, path) NULL
+#endif
+
 #ifdef __TANDEM
 #include <floss.h(floss_execl,floss_execlp,floss_execv,floss_execvp)>
 #include <floss.h(floss_getpwuid)>