test-lib: simplify '--option=value' parsing

To get the 'value' from '--option=value', test-lib.sh parses said
option running 'expr' with a regexp.  This involves a subshell, an
external process, and a lot of non-alphanumeric characters in the
regexp.

Use a much simpler POSIX-defined shell parameter expansion instead to
do the same.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -202,13 +202,13 @@ do
 		}
 		run_list=$1; shift ;;
 	--run=*)
-		run_list=$(expr "z$1" : 'z[^=]*=\(.*\)'); shift ;;
+		run_list=${1#--*=}; shift ;;
 	-h|--h|--he|--hel|--help)
 		help=t; shift ;;
 	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
 		verbose=t; shift ;;
 	--verbose-only=*)
-		verbose_only=$(expr "z$1" : 'z[^=]*=\(.*\)')
+		verbose_only=${1#--*=}
 		shift ;;
 	-q|--q|--qu|--qui|--quie|--quiet)
 		# Ignore --quiet under a TAP::Harness. Saying how many tests
@@ -222,15 +222,15 @@ do
 		valgrind=memcheck
 		shift ;;
 	--valgrind=*)
-		valgrind=$(expr "z$1" : 'z[^=]*=\(.*\)')
+		valgrind=${1#--*=}
 		shift ;;
 	--valgrind-only=*)
-		valgrind_only=$(expr "z$1" : 'z[^=]*=\(.*\)')
+		valgrind_only=${1#--*=}
 		shift ;;
 	--tee)
 		shift ;; # was handled already
 	--root=*)
-		root=$(expr "z$1" : 'z[^=]*=\(.*\)')
+		root=${1#--*=}
 		shift ;;
 	--chain-lint)
 		GIT_TEST_CHAIN_LINT=1