use skip_prefix to avoid magic numbers

It's a common idiom to match a prefix and then skip past it
with a magic number, like:

  if (starts_with(foo, "bar"))
	  foo += 3;

This is easy to get wrong, since you have to count the
prefix string yourself, and there's no compiler check if the
string changes.  We can use skip_prefix to avoid the magic
numbers here.

Note that some of these conversions could be much shorter.
For example:

  if (starts_with(arg, "--foo=")) {
	  bar = arg + 6;
	  continue;
  }

could become:

  if (skip_prefix(arg, "--foo=", &bar))
	  continue;

However, I have left it as:

  if (skip_prefix(arg, "--foo=", &v)) {
	  bar = v;
	  continue;
  }

to visually match nearby cases which need to actually
process the string. Like:

  if (skip_prefix(arg, "--foo=", &v)) {
	  bar = atoi(v);
	  continue;
  }

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

alias.c

@@ -5,7 +5,8 @@ static char *alias_val;
 
 static int alias_lookup_cb(const char *k, const char *v, void *cb)
 {
-	if (starts_with(k, "alias.") && !strcmp(k + 6, alias_key)) {
+	const char *name;
+	if (skip_prefix(k, "alias.", &name) && !strcmp(name, alias_key)) {
 		if (!v)
 			return config_error_nonbool(k);
 		alias_val = xstrdup(v);