Add example for iscommit and reset! (JuliaLang#21734)

base/libgit2/libgit2.jl

@@ -72,6 +72,19 @@ end
 
 Checks if commit `id` (which is a [`GitHash`](@ref) in string form)
 is in the repository.
+
+# Example
+
+```julia
+julia> repo = LibGit2.GitRepo(repo_path);
+
+julia> LibGit2.add!(repo, test_file);
+
+julia> commit_oid = LibGit2.commit(repo, "add test_file");
+
+julia> LibGit2.iscommit(string(commit_oid), repo)
+true
+```
 """
 function iscommit(id::AbstractString, repo::GitRepo)
     res = true
@@ -573,6 +586,21 @@ set by `mode`:
   3. `Consts.RESET_HARD` - move HEAD to `id`, reset the index to `id`, and discard all working changes.
 
 Equivalent to `git reset [--soft | --mixed | --hard] <id>`.
+
+# Example
+
+```julia
+repo = LibGit2.GitRepo(repo_path)
+head_oid = LibGit2.head_oid(repo)
+open(joinpath(repo_path, "file1"), "w") do f
+    write(f, "111\n")
+end
+LibGit2.add!(repo, "file1")
+mode = LibGit2.Consts.RESET_HARD
+# will discard the changes to file1
+# and unstage it
+new_head = LibGit2.reset!(repo, head_oid, mode)
+```
 """
 reset!(repo::GitRepo, id::GitHash, mode::Cint = Consts.RESET_MIXED) =
     reset!(repo, GitObject(repo, id), mode)