fast-import: tighten path unquoting

fast-import: tighten path unquoting

Path parsing in fast-import is inconsistent and many unquoting errors
are suppressed or not checked.

<path> appears in the grammar in these places:

    filemodify ::= 'M' SP <mode> (<dataref> | 'inline') SP <path> LF
    filedelete ::= 'D' SP <path> LF
    filecopy   ::= 'C' SP <path> SP <path> LF
    filerename ::= 'R' SP <path> SP <path> LF
    ls         ::= 'ls' SP <dataref> SP <path> LF
    ls-commit  ::= 'ls' SP <path> LF

and fast-import.c parses them in five different ways:

1. For filemodify and filedelete:
   Try to unquote <path>. If it unquotes without errors, use the
   unquoted version; otherwise, treat it as literal bytes to the end of
   the line (including any number of SP).
2. For filecopy (source) and filerename (source):
   Try to unquote <path>. If it unquotes without errors, use the
   unquoted version; otherwise, treat it as literal bytes up to, but not
   including, the next SP.
3. For filecopy (dest) and filerename (dest):
   Like 1., but an unquoted empty string is forbidden.
4. For ls:
   If <path> starts with `"`, unquote it and report parse errors;
   otherwise, treat it as literal bytes to the end of the line
   (including any number of SP).
5. For ls-commit:
   Unquote <path> and report parse errors.
   (It must start with `"` to disambiguate from ls.)

In the first three, any errors from trying to unquote a string are
suppressed, so a quoted string that contains invalid escapes would be
interpreted as literal bytes. For example, `"\xff"` would fail to
unquote (because hex escapes are not supported), and it would instead be
interpreted as the byte sequence '"', '\\', 'x', 'f', 'f', '"', which is
certainly not intended. Some front-ends erroneously use their language's
standard quoting routine instead of matching Git's, which could silently
introduce escapes that would be incorrectly parsed due to this and lead
to data corruption.

The documentation states “To use a source path that contains SP the path
must be quoted.”, so it is expected that some implementations depend on
spaces being allowed in paths in the final position. Thus we have two
documented ways to parse paths, so simplify the implementation to that.

Now we have:

1. `parse_path_eol` for filemodify, filedelete, filecopy (dest),
   filerename (dest), ls, and ls-commit:

   If <path> starts with `"`, unquote it and report parse errors;
   otherwise, treat it as literal bytes to the end of the line
   (including any number of SP).

2. `parse_path_space` for filecopy (source) and filerename (source):

   If <path> starts with `"`, unquote it and report parse errors;
   otherwise, treat it as literal bytes up to, but not including, the
   next SP. It must be followed by SP.

There remain two special cases: The dest <path> in filecopy and rename
cannot be an unquoted empty string (this will be addressed subsequently)
and <path> in ls-commit must be quoted to disambiguate it from ls.

Signed-off-by: Thalia Archibald <thalia@archibald.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>