Shiloh is originally a Biblical place name, mentioned several times in the books of Joshua, Judges, and I Samuel in the Old Testament. Because of this it was used as a name for churches and towns in the USA, and that led to its becoming famous during the American Civil War because of the Battle of Shiloh.
The name was then used for the name of the ranch in the US television series The Virginian, which aired between 1962 and 1971, and turned up during that time period as the name of several male characters on other TV Westerns, it probably being assumed that these were Civil War veterans nicknamed after their fighting at the Battle of Shiloh. This began to inspire a few parents, and when Cheyenne later became popular for girls parents began to use Shiloh as a feminine form also. It's always been a rare given name, but AngelinaJolie was far from the first American parent to use it.