Allow requesting badges for branches != master

This changed the cache structure to allow caching data for multiple
branches. The service still assumes a default branch named `master` but
using the `branch` get parameter, the default branch can be changed.

The 404 page for the missing `master` branch was changed to explain the
new parameter.

src/cache.rs

@@ -1,6 +1,7 @@
 use crate::error::{Error, Result};
 use std::{
     borrow::Cow,
+    collections::HashMap,
     fs::{create_dir_all, File, OpenOptions},
     io::BufReader,
     path::Path,
@@ -9,49 +10,106 @@ use std::{
 /// Enum to indicate the state of the cache
 pub(crate) enum CacheState<'a> {
     /// Current head and cached head are the same
-    Current { count: u64, commits: u64 },
+    Current {
+        count: u64,
+        commits: u64,
+        cache: Cache<'a>,
+    },
     /// Cached head is older than current head
-    Old(Cache<'a>),
+    Old {
+        head: String,
+        cache: Cache<'a>,
+    },
+    NoneForBranch(Cache<'a>),
     /// No cache was found
     No,
 }
 
 impl<'a> CacheState<'a> {
-    pub(crate) fn read_from_file(path: impl AsRef<Path>, head: &str) -> Result<CacheState> {
+    pub(crate) fn read_from_file(
+        path: impl AsRef<Path>,
+        branch: &str,
+        head: &str,
+    ) -> Result<CacheState<'a>> {
         if path.as_ref().exists() {
             let cache: Cache = serde_json::from_reader(BufReader::new(File::open(path)?))?;
-            if cache.head == head {
-                Ok(CacheState::Current {
-                    count: cache.count,
-                    commits: cache.commits,
+            Ok(cache
+                .entries
+                .get(branch)
+                .map(|c| {
+                    if c.head == head {
+                        CacheState::Current {
+                            count: c.count,
+                            commits: c.commits,
+                            // TODO: get rid of clone
+                            cache: cache.clone(),
+                        }
+                    } else {
+                        CacheState::Old {
+                            head: c.head.to_string(),
+                            // TODO: get rid of clone
+                            cache: cache.clone(),
+                        }
+                    }
                 })
-            } else {
-                Ok(CacheState::Old(cache))
-            }
+                // TODO: get rid of clone
+                .unwrap_or_else(|| CacheState::NoneForBranch(cache.clone())))
         } else {
             Ok(CacheState::No)
         }
     }
 
-    pub(crate) fn calculate_new_cache(self, count: u64, commits: u64, head: Cow<'a, str>) -> Cache {
+    pub(crate) fn calculate_new_cache(
+        self,
+        count: u64,
+        commits: u64,
+        head: Cow<'a, str>,
+        branch: &'a str,
+    ) -> Cache<'a> {
         match self {
-            CacheState::Old(mut cache) => {
-                cache.head = head;
-                cache.count += count;
-                cache.commits += commits;
+            CacheState::Old { mut cache, .. } => {
+                if let Some(mut cache) = cache.entries.get_mut(branch) {
+                    cache.head = head;
+                    cache.count += count;
+                    cache.commits += commits;
+                }
                 cache
             }
-            CacheState::No | CacheState::Current { .. } => Cache {
-                head,
-                count,
-                commits,
-            },
+            CacheState::Current { cache, .. } => cache,
+            CacheState::NoneForBranch(mut cache) => {
+                cache.entries.insert(
+                    branch.into(),
+                    CacheEntry {
+                        head,
+                        count,
+                        commits,
+                    },
+                );
+                cache
+            }
+            CacheState::No => {
+                let mut entries = HashMap::with_capacity(1);
+                entries.insert(
+                    branch.into(),
+                    CacheEntry {
+                        commits,
+                        head,
+                        count,
+                    },
+                );
+                Cache { entries }
+            }
         }
     }
 }
 
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Clone)]
 pub(crate) struct Cache<'a> {
+    pub entries: HashMap<Cow<'a, str>, CacheEntry<'a>>,
+}
+
+#[derive(Serialize, Deserialize, Clone)]
+pub(crate) struct CacheEntry<'a> {
     /// HEAD commit ref
     pub head: Cow<'a, str>,
     /// HoC value