Merge branch 'jk/http-auth-redirects' into maint

We did not handle cases where http transport gets redirected during
the authorization request (e.g. from http:// to https://).

* jk/http-auth-redirects:
  http.c: Spell the null pointer as NULL
  remote-curl: rewrite base url from info/refs redirects
  remote-curl: store url as a strbuf
  remote-curl: make refs_url a strbuf
  http: update base URLs when we see redirects
  http: provide effective url to callers
  http: hoist credential request out of handle_curl_result
  http: refactor options to http_get_*
  http_request: factor out curlinfo_strbuf
  http_get_file: style fixes
Merge branch 'jc/upload-pack-send-symref' into maint

One long-standing flaw in the pack transfer protocol used by "git
clone" was that there was no way to tell the other end which branch
"HEAD" points at, and the receiving end needed to guess.  A new
capability has been defined in the pack protocol to convey this
information so that cloning from a repository with more than one
branches pointing at the same commit where the HEAD is at now
reliably sets the initial branch in the resulting repository.

* jc/upload-pack-send-symref:
  t5570: Update for clone-progress-to-stderr branch
  t5570: Update for symref capability
  clone: test the new HEAD detection logic
  connect: annotate refs with their symref information in get_remote_head()
  connect.c: make parse_feature_value() static
  upload-pack: send non-HEAD symbolic refs
  upload-pack: send symbolic ref information as capability
  upload-pack.c: do not pass confusing cb_data to mark_our_ref()
  t5505: fix "set-head --auto with ambiguous HEAD" test
Merge branch 'ap/remote-hg-unquote-cquote' into maint

A fast-import stream expresses a pathname with funny characters by
quoting them in C style; remote-hg remote helper (in contrib/)
forgot to unquote such a path.

* ap/remote-hg-unquote-cquote:
  remote-hg: unquote C-style paths when exporting
Merge branch 'jn/test-prereq-perl-doc' into maint

The interaction between use of Perl in our test suite and NO_PERL
has been clarified a bit.

* jn/test-prereq-perl-doc:
  t/README: tests can use perl even with NO_PERL
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