remote.c: don't BUG() on 0-length branch names

remote.c: don't BUG() on 0-length branch names

4a2dcb1a08 (remote: die if branch is not found in repository,
2021-11-17) introduced a regression where multiple config entries with
an empty branch name, e.g.

[branch ""]
  remote = foo
  merge = bar

could cause Git to fail when it tries to look up branch tracking
information.

We parse the config key to get (branch name, branch name length), but
when the branch name subsection is empty, we get a bogus branch name,
e.g. "branch..remote" gives (".remote", 0). We continue to use the bogus
branch name as if it were valid, and prior to 4a2dcb1a08, this wasn't an
issue because length = 0 caused the branch name to effectively be ""
everywhere.

However, that commit handles length = 0 inconsistently when we create
the branch:

- When find_branch() is called to check if the branch exists in the
  branch hash map, it interprets a length of 0 to mean that it should
  call strlen on the char pointer.
- But the code path that inserts into the branch hash map interprets a
  length of 0 to mean that the string is 0-length.

This results in the bug described above:

- "branch..remote" looks for ".remote" in the branch hash map. Since we
  do not find it, we insert the "" entry into the hash map.
- "branch..merge" looks for ".merge" in the branch hash map. Since we
  do not find it, we again try to insert the "" entry into the hash map.
  However, the entries in the branch hash map are supposed to be
  appended to, not overwritten.
- Since overwriting an entry is a BUG(), Git fails instead of silently
  ignoring the empty branch name.

Fix the bug by removing the convenience strlen functionality, so that
0 means that the string is 0-length. We still insert a bogus branch name
into the hash map, but this will be fixed in a later commit.

Reported-by: "Ing. Martin Prantl Ph.D." <perry@ntis.zcu.cz>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>