line-range: fix infinite loop bug with '$' regex

When the -L argument to "git log" is passed the zero-width regular
expression "$" (as in "-L :$:line-range.c"), this results in an
infinite loop in find_funcname_matching_regexp().

Modify find_funcname_matching_regexp to correctly match the entire line
instead of the zero-width match at eol and update the loop condition to
prevent an infinite loop in the event of other undiscovered corner cases.

The primary change is that we pre-decrement the beginning-of-line marker
('bol') before comparing it to '\n'. In the case of '$', where we match the
'\n' at the end of the line and start the loop with bol == eol, this
ensures that bol will find the beginning of the line on which the match
occurred.

Originally reported in <https://stackoverflow.com/q/74690545/147356>.

Signed-off-by: Lars Kellogg-Stedman <lars@oddbit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4211-line-log.sh

@@ -315,4 +315,26 @@ test_expect_success 'line-log with --before' '
 	test_cmp expect actual
 '
 
+test_expect_success 'setup tests for zero-width regular expressions' '
+	cat >expect <<-EOF
+	Modify func1() in file.c
+	Add func1() and func2() in file.c
+	EOF
+'
+
+test_expect_success 'zero-width regex $ matches any function name' '
+	git log --format="%s" --no-patch "-L:$:file.c" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'zero-width regex ^ matches any function name' '
+	git log --format="%s" --no-patch "-L:^:file.c" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'zero-width regex .* matches any function name' '
+	git log --format="%s" --no-patch "-L:.*:file.c" >actual &&
+	test_cmp expect actual
+'
+
 test_done