Add the possibility to rename abbreviations

The abbr function doesn't have the possiblity to rename abbreviations.
You have to delete the old one and create a new one. This commit adds
this functionality and uses the syntax:

abbr -r OLD_KEY NEW_KEY

Fixes #2155.

doc_src/abbr.txt

@@ -3,6 +3,7 @@
 \subsection abbr-synopsis Synopsis
 \fish{synopsis}
 abbr --add word phrase...
+abbr --rename word new_word
 abbr --show
 abbr --list
 abbr --erase word
@@ -33,6 +34,8 @@ The following parameters are available:
 
 - `-a WORD PHRASE` or `--add WORD PHRASE` Adds a new abbreviation, causing WORD to be expanded to PHRASE.
 
+- `-r WORD NEW_WORD` or `--rename WORD NEW_WORD` Renames an abbreviation, from WORD to NEW_WORD.
+
 - `-s` or `--show` Show all abbreviated words and their expanded phrases in a manner suitable for export and import.
 
 - `-l` or `--list` Lists all abbreviated words.
@@ -48,6 +51,11 @@ abbr -a gco git checkout
 \endfish
 Add a new abbreviation where `gco` will be replaced with `git checkout`.
 
+\fish
+abbr -r gco gch
+\endfish
+Renames an existing abbreviation from `gco` to `gch`.
+
 \fish
 abbr -e gco
 \endfish