diagnostics: copy GCC9's format

Currently, when we quote the source file, we indent it with one space,
and preserve tabulations, so there is a discrepancy and the visual
rendering is bad.  One way out is to indent with a tab instead of a
space, but then this space can be used for more information.  This is
what GCC9 does.  Let's play copy cats.

See
https://lists.gnu.org/archive/html/bison-patches/2019-04/msg00025.html
https://developers.redhat.com/blog/2019/03/08/usability-improvements-in-gcc-9/
https://gcc.gnu.org/onlinedocs/gccint/Guidelines-for-Diagnostics.html#Guidelines-for-Diagnostics

* src/location.c (location_caret): Prefix quoted lines with the line
number and a pipe, fitting 8 columns.

* tests/actions.at, tests/c++.at, tests/conflicts.at,
* tests/diagnostics.at, tests/input.at, tests/java.at,
* tests/named-refs.at, tests/reduce.at, tests/regression.at,
* tests/sets.at: Adjust expectations.
Partly by "./build-aux/update-test tests/testsuite.dir/*/testsuite.log"
repeatedly, and partly by hand.

tests/c++.at

@@ -248,14 +248,14 @@ exp:
 
 AT_BISON_CHECK([[-fcaret input.yy]], [0], [],
 [[input.yy:16.33-34: warning: multiple occurrences of $2 with api.value.automove [-Wother]
- | "twice" exp       { $$ = $2 + $2; }
-                                 ^~
+   16 | | "twice" exp       { $$ = $2 + $2; }
+      |                                 ^~
 input.yy:17.33-36: warning: multiple occurrences of $2 with api.value.automove [-Wother]
- | "thrice" exp[val] { $$ = $2 + $val + $2; }
-                                 ^~~~
+   17 | | "thrice" exp[val] { $$ = $2 + $val + $2; }
+      |                                 ^~~~
 input.yy:17.40-41: warning: multiple occurrences of $2 with api.value.automove [-Wother]
- | "thrice" exp[val] { $$ = $2 + $val + $2; }
-                                        ^~
+   17 | | "thrice" exp[val] { $$ = $2 + $val + $2; }
+      |                                        ^~
 ]])
 
 AT_BISON_OPTION_POPDEFS