cgit.c: Do not reset HOME after unsetting it.

The number of odd cases in which git will try to read config is far too
great to keep putting a bandaid over each one, so we'll just unset it.

If it turns out that scripts really liked to know about $HOME, we can
always reset it in the filter forks.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
ui-summary: Use default branch for readme if : prefix

If the readme value begins with ":", and has no specified branch before
it, use the repository's default branch.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
ui-summary: Pass filename to about-filter

This gives the about-filter API the same semantics as source-filter,
where the filter receives the filename so it can decide what to do next
with it.

While we're at it, plug a memory leak.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
readme: Accept multiple candidates and test them.

The readme variable may now contain multiple space deliminated entries,
which per usual are either a filepath or a git ref filepath. If multiple
are specified, cgit will now select the first one in the list that
exists. This is to make it easier to specify multiple default readme
types in the main cgitrc file and have them automatically get applied to
each repo based on what exists.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
cgitrc.5: information on directory traversal and multiple readme files

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
ui-summary: Disallow directory traversal

Using the url= query string, it was possible request arbitrary files
from the filesystem if the readme for a given page was set to a
filesystem file. The following request would return my /etc/passwd file:

http://git.zx2c4.com/?url=/somerepo/about/../../../../etc/passwd
http://data.zx2c4.com/cgit-directory-traversal.png

This fix uses realpath(3) to canonicalize all paths, and then compares
the base components.

This fix introduces a subtle timing attack, whereby a client can check
whether or not strstr is called using timing measurements in order
to determine if a given file exists on the filesystem.

This fix also does not account for filesystem race conditions (TOCTOU)
in resolving symlinks.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
readme: use string_list instead of space deliminations

Now this is possible in cgitrc -

readme=:README.md
readme=:readme.md
readme=:README.mkd
readme=:readme.mkd
readme=:README.rst
readme=:readme.rst
readme=:README.html
readme=:readme.html
readme=:README.htm
readme=:readme.htm
readme=:README.txt
readme=:readme.txt
readme=:README
readme=:readme
readme=:INSTALL.txt
readme=:install.txt
readme=:INSTALL
readme=:install

Suggested-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filters: import more modern scripts

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
cgitrc.5: improve example config

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
README: add trailing slash to homepage

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>