Memoize result value in Utilities.getremote

Utilities.getremote is called when figuring out the URL to methods while
rendering the documentation. A profile of makedocs for Documenters docs
showed that almost half of the collected samples was inside getremote,
and in particular in the spawned process to git. This patch memoizes the
result of getremote. This should give noticeable improvements, since
this function is often often called with the same argument.

src/Utilities/Utilities.jl

@@ -512,19 +512,17 @@ function url(remote, repo, mod, file, linerange)
     end
 end
 
+const GIT_REMOTE_CACHE = Dict{String,String}()
+
 function getremote(dir::AbstractString)
-    remote =
-        try
-            cd(() -> readchomp(`git config --get remote.origin.url`), dir)
-        catch err
+    return get!(GIT_REMOTE_CACHE, dir) do
+        remote = try
+            readchomp(setenv(`git config --get remote.origin.url`; dir=dir))
+        catch
             ""
         end
-    m = match(LibGit2.GITHUB_REGEX, remote)
-    if m === nothing
-        travis = get(ENV, "TRAVIS_REPO_SLUG", "")
-        isempty(travis) ? "" : travis
-    else
-        m[1]
+        m = match(LibGit2.GITHUB_REGEX, remote)
+        return m === nothing ? get(ENV, "TRAVIS_REPO_SLUG", "") : String(m[1])
     end
 end