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

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>