Fix spelling of committish

The official git glossary (https://git-scm.com/docs/gitglossary),
spells this commit-ish or committish. Since we can't do the former,
use the latter consistently. We had three different spellings.

stdlib/LibGit2/src/LibGit2.jl

@@ -728,7 +728,7 @@ function merge!(repo::GitRepo;
                                "There is no fetch reference for this branch."))
             end
             Base.map(fh->GitAnnotated(repo,fh), fheads)
-        else # merge commitish
+        else # merge committish
             [GitAnnotated(repo, committish)]
         end
     else

stdlib/LibGit2/src/merge.jl

@@ -4,7 +4,7 @@
     GitAnnotated(repo::GitRepo, commit_id::GitHash)
     GitAnnotated(repo::GitRepo, ref::GitReference)
     GitAnnotated(repo::GitRepo, fh::FetchHead)
-    GitAnnotated(repo::GitRepo, comittish::AbstractString)
+    GitAnnotated(repo::GitRepo, committish::AbstractString)
 
 An annotated git commit carries with it information about how it was looked up and
 why, so that rebase or merge operations have more information about the context of
@@ -40,8 +40,8 @@ function GitAnnotated(repo::GitRepo, fh::FetchHead)
     return GitAnnotated(repo, ann_ref_ref[])
 end
 
-function GitAnnotated(repo::GitRepo, comittish::AbstractString)
-    obj = GitObject(repo, comittish)
+function GitAnnotated(repo::GitRepo, committish::AbstractString)
+    obj = GitObject(repo, committish)
     cmt = peel(GitCommit, obj)
     return GitAnnotated(repo, GitHash(cmt))
 end

stdlib/LibGit2/src/repository.jl

@@ -263,32 +263,32 @@ end
 peel(obj::GitObject) = peel(GitObject, obj)
 
 """
-    LibGit2.GitDescribeResult(commitish::GitObject; kwarg...)
+    LibGit2.GitDescribeResult(committish::GitObject; kwarg...)
 
-Produce a `GitDescribeResult` of the `commitish` `GitObject`, which
+Produce a `GitDescribeResult` of the `committish` `GitObject`, which
 contains detailed information about it based on the keyword argument:
 
   * `options::DescribeOptions=DescribeOptions()`
 
-A git description of a `commitish` object looks for the tag (by default, annotated,
-although a search of all tags can be performed) which can be reached from `commitish`
-which is most recent. If the tag is pointing to `commitish`, then only the tag is
+A git description of a `committish` object looks for the tag (by default, annotated,
+although a search of all tags can be performed) which can be reached from `committish`
+which is most recent. If the tag is pointing to `committish`, then only the tag is
 included in the description. Otherwise, a suffix is included which contains the
-number of commits between `commitish` and the most recent tag. If there is no such
+number of commits between `committish` and the most recent tag. If there is no such
 tag, the default behavior is for the description to fail, although this can be
 changed through `options`.
 
-Equivalent to `git describe <commitish>`. See [`DescribeOptions`](@ref) for more
+Equivalent to `git describe <committish>`. See [`DescribeOptions`](@ref) for more
 information.
 """
-function GitDescribeResult(commitish::GitObject;
+function GitDescribeResult(committish::GitObject;
                            options::DescribeOptions=DescribeOptions())
     ensure_initialized()
     result_ptr_ptr = Ref{Ptr{Cvoid}}(C_NULL)
     @check ccall((:git_describe_commit, :libgit2), Cint,
                  (Ptr{Ptr{Cvoid}}, Ptr{Cvoid}, Ptr{DescribeOptions}),
-                 result_ptr_ptr, commitish.ptr, Ref(options))
-    return GitDescribeResult(commitish.owner, result_ptr_ptr[])
+                 result_ptr_ptr, committish.ptr, Ref(options))
+    return GitDescribeResult(committish.owner, result_ptr_ptr[])
 end
 
 """