t/perf: fix test_export() failure with BSD `sed`

test_perf() runs each test in its own subshell which makes it difficult to persist variables between tests. test_export() addresses this shortcoming by grabbing the values of specified variables after a test runs but before the subshell exits, and writes those values to a file which is loaded into the environment of subsequent tests. To grab the values to be persisted, test_export() pipes the output of the shell's builtin `set` command through `sed` which plucks them out using a regular expression along the lines of `s/^(var1|var2)/.../p`. Unfortunately, though, this use of alternation is not portable. For instance, BSD-lineage `sed` (including macOS `sed`) does not support it in the default "basic regular expression" mode (BRE). It may be possible to enable "extended regular expression" mode (ERE) in some cases with `sed -E`, however, `-E` is neither portable nor part of POSIX. Fortunately, alternation is unnecessary in this case and can easily be avoided, so replace it with a series of simple expressions such as `s/^var1/.../p;s/^var2/.../p`. While at it, tighten the expressions so they match the variable names exactly rather than matching prefixes (i.e. use `s/^var1=/.../p`). If the requirements of test_export() become more complex in the future, then an alternative would be to replace `sed` with `perl` which supports alternation on all platforms, however, the simple elimination of alternation via multiple `sed` expressions suffices for the present. Reported-by: Sangeeta <sangunb09@gmail.com> Diagnosed-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>