t2400, t5516: strip directory prefix from --git-dir path arguments

This commit replaces a couple of `-C <dir>` invocations where
`<dir>` is a bare repository with `--git-dir=<dir>` as part of the
`safe.bareRepository` preparation.

Since `--git-dir` does not change the working directory, paths that
duplicated the directory name as a prefix are now redundant.

This commit can be validated in a mechanical way via this command-line:

  git diff HEAD^! | awk '
    /^diff/,/^\+\+\+/ { next }
    /^-/ { old[m++] = substr($0, 2) }
    /^\+/ {
      line = substr($0,2)
      # apply: -C X -> --git-dir=X and strip ../
      if (match(line, /--git-dir=([^ ]*)(.* )([^ ]*)\//, a) &&
           a[1] == a[3]) {
        gsub(/--git-dir=.*\//, "-C " a[1] a[2], line)
      }
      if (old[n++] != line) print old[n-1] " != " line
    }'

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t2400-worktree-add.sh

@@ -397,7 +397,7 @@ test_expect_success '"add --orphan" with empty repository' '
 	test_when_finished "rm -rf empty_repo" &&
 	echo refs/heads/newbranch >expected &&
 	GIT_DIR="empty_repo" git init --bare &&
-	git -C empty_repo worktree add --orphan -b newbranch worktreedir &&
+	git --git-dir=empty_repo worktree add --orphan -b newbranch empty_repo/worktreedir &&
 	git -C empty_repo/worktreedir symbolic-ref HEAD >actual &&
 	test_cmp expected actual
 '

t/t5516-fetch-push.sh

@@ -1806,7 +1806,7 @@ test_expect_success 'denyCurrentBranch and worktrees' '
 test_expect_success 'denyCurrentBranch and bare repository worktrees' '
 	test_when_finished "rm -fr bare.git" &&
 	git clone --bare . bare.git &&
-	git -C bare.git worktree add wt &&
+	git --git-dir=bare.git worktree add bare.git/wt &&
 	test_commit grape &&
 	git -C bare.git config receive.denyCurrentBranch refuse &&
 	test_must_fail git push bare.git HEAD:wt &&
@@ -1827,7 +1827,7 @@ test_expect_success 'refuse fetch to current branch of worktree' '
 test_expect_success 'refuse fetch to current branch of bare repository worktree' '
 	test_when_finished "rm -fr bare.git" &&
 	git clone --bare . bare.git &&
-	git -C bare.git worktree add wt &&
+	git --git-dir=bare.git worktree add bare.git/wt &&
 	test_commit banana &&
 	test_must_fail git -C bare.git fetch .. HEAD:wt &&
 	git -C bare.git fetch -u .. HEAD:wt