Re: [PATCH 10/16] fast-import: use skip_prefix for parsing input

Fast-import does a lot of parsing of commands and
dispatching to sub-functions. For example, given "option
foo", we might recognize "option " using starts_with, and
then hand it off to parse_option() to do the rest.

However, we do not let parse_option know that we have parsed
the first part already. It gets the full buffer, and has to
skip past the uninteresting bits. Some functions simply add
a magic constant:

  char *option = command_buf.buf + 7;

Others use strlen:

  char *option = command_buf.buf + strlen("option ");

And others use strchr:

  char *option = strchr(command_buf.buf, ' ') + 1;

All of these are brittle and easy to get wrong (especially
given that the starts_with call and the code that assumes
the presence of the prefix are far apart). Instead, we can
use skip_prefix, and just pass each handler a pointer to its
arguments.

Signed-off-by: Jeff King <redacted>
---

diff --git a/fast-import.c b/fast-import.c
index a3ffe30..5f17adb 100644
--- a/fast-import.c
+++ b/fast-import.c

@@ -2713,20 +2706,22 @@ static void parse_new_commit(void)

        /* file_change* */
        while (command_buf.len > 0) {
-               if (starts_with(command_buf.buf, "M "))
-                       file_change_m(b);
-               else if (starts_with(command_buf.buf, "D "))
-                       file_change_d(b);
-               else if (starts_with(command_buf.buf, "R "))
-                       file_change_cr(b, 1);
-               else if (starts_with(command_buf.buf, "C "))
-                       file_change_cr(b, 0);
-               else if (starts_with(command_buf.buf, "N "))
-                       note_change_n(b, &prev_fanout);
+               const char *v;