argv: tweak the meaning of zero

This commit makes a small tweak to the --max-columns flag. Namely, if
the value of the flag is 0, then ripgrep behaves as-if the flag were
absent.

This is useful in the context of ripgrep reading configuration from the
environment. For example, an end user might set --max-columns=150, but we
should permit the user to disable this setting when needed. Using -M0 is
a nice way to do that.

We do this because a zero value for --max-columns isn't particularly
meaningful. We do leave the --max-count, --max-filesize and --maxdepth
flags alone though, since a zero value for those flags is potentially
meaningful. (--max-count even has tests for ripgrep's behavior when
given a value of 0.)

src/args.rs

@@ -381,8 +381,8 @@ impl<'a> ArgMatches<'a> {
             line_number: line_number,
             line_number_width: try!(self.usize_of("line-number-width")),
             line_per_match: self.is_present("vimgrep"),
-            max_columns: self.usize_of("max-columns")?,
-            max_count: self.usize_of("max-count")?.map(|max| max as u64),
+            max_columns: self.usize_of_nonzero("max-columns")?,
+            max_count: self.usize_of("max-count")?.map(|n| n as u64),
             max_filesize: self.max_filesize()?,
             maxdepth: self.usize_of("maxdepth")?,
             mmap: mmap,
@@ -971,6 +971,24 @@ impl<'a> ArgMatches<'a> {
         self.values_of_lossy(name).unwrap_or_else(Vec::new)
     }
 
+    /// Safely reads an arg value with the given name, and if it's present,
+    /// tries to parse it as a usize value.
+    ///
+    /// If the number is zero, then it is considered absent and `None` is
+    /// returned.
+    fn usize_of_nonzero(&self, name: &str) -> Result<Option<usize>> {
+        match self.value_of_lossy(name) {
+            None => Ok(None),
+            Some(v) => v.parse().map_err(From::from).map(|n| {
+                if n == 0 {
+                    None
+                } else {
+                    Some(n)
+                }
+            }),
+        }
+    }
+
     /// Safely reads an arg value with the given name, and if it's present,
     /// tries to parse it as a usize value.
     fn usize_of(&self, name: &str) -> Result<Option<usize>> {