box: make --box-commit-retries flag defaulting to 100 - Fixes #2054

Sometimes it takes many more commit retries than expected to commit a
multipart file, so split this number into its own config variable and
default it to 100 which should always be enough.

docs/content/box.md

@@ -227,6 +227,10 @@ system.
 Cutoff for switching to chunked upload - must be >= 50MB. The default
 is 50MB.
 
+#### --box-commit-retries int ####
+
+Max number of times to try committing a multipart file. (default 100)
+
 ### Limitations ###
 
 Note that Box is case insensitive so you can't have a file called