grep: fix match highlighting for combined patterns with context lines

When git grep is run with combined patterns such as '-e p1 --and -e
p2' and surrounding context lines are requested, the output contains
incorrectly highlighted matches.

Consider the following output (highlighted matches are surrounded by '*'
characters):
    $ cat testfile
    foo a
    foo b
    foo bar
    baz bar foo
    bar x
    bar y
    $ git grep -n -C2 -e foo --and -e bar testfile
    testfile-1-*foo* a
    testfile-2-*foo* b
    testfile:3:*foo* *bar*
    testfile:4:baz *bar* *foo*
    testfile-5-*bar* x
    testfile-6-*bar* y

Lines 1, 2, 5 and 6 do not match the combined patterns, they only
contain incorrectly highlighted 'false positives'.

Modify the show_line() function in grep.c to highlight matches only on
lines that match the combined pattern. Do not highlight matches on lines
that provide only context or contain only the function name of the match.

The output of the same command after the change:
    $ git grep -n -C2 -e foo --and -e bar testfile
    testfile-1-foo a
    testfile-2-foo b
    testfile:3:*foo* *bar*
    testfile:4:baz *bar* *foo*
    testfile-5-bar x
    testfile-6-bar y

Also avoid unnecessary regmatch on lines we do not split by
moving the code to paint a single line piece-by-piece only
to apply to the matched line.

Signed-off-by: Zoltan Klinger <zoltan.klinger@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>