wt-status: correct and simplify check for detached HEAD

wt-status: correct and simplify check for detached HEAD

If a branch name is longer than four characters then memcmp() reads over
the end of the static string "HEAD".  This causes the following test
failures with AddressSanitizer:

t3203-branch-output.sh                           (Wstat: 256 Tests: 18 Failed: 4)
  Failed tests:  12, 15-17
  Non-zero exit status: 1
t3412-rebase-root.sh                             (Wstat: 256 Tests: 31 Failed: 3)
  Failed tests:  28-29, 31
  Non-zero exit status: 1
t3507-cherry-pick-conflict.sh                    (Wstat: 256 Tests: 31 Failed: 4)
  Failed tests:  14, 29-31
  Non-zero exit status: 1
t3510-cherry-pick-sequence.sh                    (Wstat: 256 Tests: 39 Failed: 14)
  Failed tests:  17, 22-26, 28-30, 34-35, 37-39
  Non-zero exit status: 1
t3420-rebase-autostash.sh                        (Wstat: 256 Tests: 28 Failed: 4)
  Failed tests:  24-27
  Non-zero exit status: 1
t3404-rebase-interactive.sh                      (Wstat: 256 Tests: 91 Failed: 57)
  Failed tests:  17, 19, 21-42, 44, 46-74, 77, 81-82
  Non-zero exit status: 1
t3900-i18n-commit.sh                             (Wstat: 256 Tests: 34 Failed: 1)
  Failed test:  34
  Non-zero exit status: 1
t5407-post-rewrite-hook.sh                       (Wstat: 256 Tests: 14 Failed: 6)
  Failed tests:  9-14
  Non-zero exit status: 1
t7001-mv.sh                                      (Wstat: 256 Tests: 46 Failed: 5)
  Failed tests:  39-43
  Non-zero exit status: 1
t7509-commit.sh                                  (Wstat: 256 Tests: 12 Failed: 2)
  Failed tests:  11-12
  Non-zero exit status: 1
t7512-status-help.sh                             (Wstat: 256 Tests: 39 Failed: 35)
  Failed tests:  5-39
  Non-zero exit status: 1
t6030-bisect-porcelain.sh                        (Wstat: 256 Tests: 70 Failed: 1)
  Failed test:  13
  Non-zero exit status: 1

And if a branch is named "H", "HE", or "HEA" then the current if clause
erroneously considers it as matching "HEAD" because it only compares
up to the end of the branch name.

Fix that by doing the comparison using strcmp() and only after the
branch name is extracted.  This way neither too less nor too many
characters are checked.  While at it call strchrnul() to find the end
of the branch name instead of open-coding it.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Jeff King <peff@peff.net>